tl;dr It would be good to distinguish between blind rulers in the cultural domain of European (post-Roman) Christianity and blind rulers elsewhere. There were an awful lot of blind European monarchs. This is due to the fact that rivals were often (very, very often) blinded to make them unfit to rule and occasionally ascended to the throne anyway.
On a different note, I agree strongly with @ed.hank that pre-modern rulers were probably always ruthless by the definition of the OP, because the OP bases the question on modern ethics that are anachronistic when applied to pre-modern times.
I will address the two issues one after the other.
Being blind or being blinded
In Christian Europe, rulers were seen as god's chosen leaders; they would be expected to serve as an example of flawless beauty. This is because according to the Bible, man was created by god in his own image (hence flawlessly beautiful) and the further you were from flawless beauty, the less godly you would be perceived as. Eyes, as a symbol of wisdom were particularly important.
For European Christian's obsession with flawlessness, compare also the veneration of virgin Mary. On some level, this veneration of beauty and flawlessness probably also had roots in ancient Greek art.
Now, if you were a contender to the throne in a medieval European country, you first and foremostly had to get rid of rivals. This included children and infants. A common way to do that without being perceived as a murderer of innocent children, was to have them blinded and placed in a monastery (so that they would 1. not have children of their own, 2. receive an education occupied with religious and politically unimportant matters, and 3. be well-guarded). This procedure was especially popular in the Byzantine empire. There are many examples for this, including some that were named in the other answers; see below.
Of course, often, the blinded successors to the thrones still had supporters and would at some point succeed to the throne anyway, if those supporters were victorious.
In non-European contexts, blindness did not have the same connotation and while blind people may have been perceived as physically impaired this would not be related to whether or not they were fit to rule. Hence, the blinding of contenders to the throne was much less commonplace (you would rather have them killed, castrated, or something like that); consequently less many successors to the throne were blind and there were less many blind monarchs.
Blind rulers in Christian Europe
Many have been mentioned in the OP or in various other answers: John of Bohemia, doge Enrico Dandolo, emperor Louis III (Louis the Blind), Béla II of Hungary, Tsar Simeon, Grand Prince Vasily II
There are many others: emperor Isaac Angelos, Simon Gurieli, Stefan Branković of Serbia, Prince Almos of Hungary (Béla II's father), Theodosius III of Abkhazia.
All of them were blinded on purpose, not due to natural causes. The list of politically mutilated figures in Byzantine politics on wikipedia has many more examples.
While this may suggest that removing rivals to the throne by blinding them was ineffective, since all cited examples eventually took the throne anyway, this is a sampling bias. It was very effective; the blinded usually never got back into politics again if they did not die of their injury directly. Consider some examples that did not return to the throne: Byzantine emperors Alexios Doukas, Leo Phokas, and Romanos Diogenes, king Magnus of Norway, king Bernhard of Italy, duke Boleslaus of Bohemia and Ilias of Moldavia. There are many more.
Blind rulers elsewhere
Elsewhere, blind rulers would generally be blind due to natural causes. This may include people who were born blind as well as people who became blind in their old age or illness.
An example for rulers who were blind when they were old is Tamerlan, the Mongol leader who almost destroyed the Ottomans half a century before they founded their empire. He led his army into his most briliant battle when he was already practically blind.
I struggle to find examples for rulers who were born blind. The ancient Indian king Dhritarashtra may be one example, but his historicity is disputed.
Ruthlessness
It is difficult to apply modern ethical interpretations to pre-modern contexts. The conditions you cite (mass murder, assassinations, execution of civilians, burning of witches, war crimes) were not generally seen as wrong (although some of them were at one time or the other).
To give you some examples:
- Burning of witches at the stake was generally done according to lawful court proceedings and would have been seen as completely lawful.
- Conquering armies were generally accepted to be entitled to the spoils of war. In particular, they saw themselves as entitled to the spoils of war and history, you may have noticed, is written by the victors. That is not to say that tales of the cruelty of opponents (whether true or false) would not be used for political propaganda.
- There were no criminal investigations in the modern sense. The purpose of government was not primarily to be just but to maintain order. If some crime had happened, the most important thing was that someone would be punished for it. There were no medieval Sherlock-Holmes like figures going around towns striving to solve murder cases; instead they would round up some of the usual suspects and torture them until they confessed, then execute them. This was seen as completely lawful and just.
Note that this does not only concern which legal proceedings would be technically lawful. This also dominated contemporary perceptions of law and justice and the education of monarchs and citizens alike. They would not just follow these procedures, they would actually believe that that was right.
For details, you may want to have a look at analyses of pre-modern systems of law and governance, e.g. Michel Foucault's "Discipline and Punish", although this study (being the first in this field) is quite old and perhaps not quite up to date.
Whether monarchs came to be seen as ruthless more often than not had to do with who dominated the historiography about their reign. (And to what extend they pissed off the historiographers during their lifetime.) Take the Mongol invasions of Europe in the early 1240s as an example:
- From the European perspective they were seen as exceptionally cruel (and godless etc)
- From the Mongol perspective they saw themselves as fulfilling the destiny of the Great Khan who was the rightful ruler of the world. If Europeans resisted the Mongol armies, was it not their own fault if they were treated cruelly?
- From the perspective of Mongol commanders, it was all strategy. Be lenient towards the people you think you can work with in the future (including Christian princes like Alexander Nevsky). But set an example to everyone who will oppose you by not only being as cruel as possible, but also making sure that reports of that cruelty were disseminated as widely as possible. And by all means engage in respectful diplomacy with the Vatican.
- From the perspective of European leaders, it was all strategy too. Use reports of the cruelty of the Mongols for propaganda and to mobilize forces against them. Unless you want to work with the Mongols because you need them to participate in a crusade. And by all means engage in respectful diplomacy with the Great Khan.
