The Basilica of the Holy Blood, a church in Bruges, claims they hold the Holy Blood and the main thesis is that it was brought to Bruges by the Second Crusade. Note that in 1964, Nicolas Huyghebaert published a study in which he claims that the Holy Blood was brought to Bruges later.
My question is not whether it is indeed the Holy Bood. My question is more whether something is known about what made the Crusaders believe that it was indeed the Holy Blood (and how they "convinced" the locals that they indeed brought back the Holy Blood). Of course the odds are high this is the blood of some knight who participated in the Crusade, but in that case he still had to convince his fellow Crusaders that it was the blood of Christ.