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The Whig connection with blue was still evident in the early 18th century, but it was evolving into blue and buff:

...as early as 1715 the diehard Bishop Burnet was referred to as a “true blue Whig” upon his death, and by mid-century the combination of buff and blue was widely associated with the Whigs.

The Whig connection with blue was still evident in the early 18th century, but [it was evolving into buff] in some regions at least. One observer of the 1705 election in Honiton noted:

the two parties were very nicely distinguished . . . Buff was the symbol of the Whigs. These had box in their hats, doors and windows; the other [Tories] had laurel leaves

Whigs were also using blue and buff:

...as early as 1715 the diehard Bishop Burnet was referred to as a “true blue Whig” upon his death, and by mid-century the combination of buff and blue was widely associated with the Whigs.

Around the same time, there were

...Tories in Newcastle-upon-Tyne in 1710...with ‘red and blue favours in their hats...

Blue was adopted by some Tories (later to become formally known as Conservatives) from around the middle of the 18th century. Before that (from around 1680) it was used mostly by Whigs. However, it's important to note that were many changes in party colours over time and, also, regional variations. Blue was not formally by the Conservative party until 1949.

When celebrations were called for upon the recovery of George III's health in 1789, the more extreme Whigs complied, but with 'message':

Blue was adopted by some Tories (later to become formally known as Conservatives) from around the middle of the 18th century. Before that (from around 1680) it was used mostly by Whigs. However, it's important to note that were many changes in party colours over time and, also, regional variations. Blue was not formally adopted by the Conservative party until 1949.

When celebrations were called for upon the recovery of George III's health in 1789, the more extreme Whigs complied, but with a 'message':

The use of blue in British party politics has a long complicated history dating back to at least the Exclusion Crisis of 1679-81 when it was associated with the earliest Whigs who wanted to exclude James II from the throne on the grounds that he was Catholic:

Short answer

Blue was adopted by some Tories (later to become formally known as Conservatives) from around the middle of the 18th century. Before that (from around 1680) it was used mostly by Whigs. However, it's important to note that were many changes in party colours over time and, also, regional variations. Blue was not formally by the Conservative party until 1949.

Details

The use of blue in British party politics has a long complicated history dating back to at least the Exclusion Crisis of 1679-81 when it was associated with the earliest Whigs who wanted to exclude James II from the throne on the grounds that he was Catholic: