Her body was found in a pool of blood.

blood | meaning of blood in Longman Dictionary of Contemporary English | LDOCE
Blood is uncountable as per dictionary then why have they used used "a pool of blood"?

It's the same as "a pile of sand" or "an armful of firewood". That's how you do it with uncountables. You could have a pile of rocks or an armful of sticks, and those are countable and take a plural.