I had heard that it was possible to taint water with blood, and experimented with different approaches trying to do this. Since dropping blood and water in the same place resulted in different pools, I thought that maybe blood had to be added to a 'Water [#/7]' location in order to taint it instead of forming a separate pool.

My experimentation, however, showed that even dropped water forms a separate pool within Water. To observe, load the below save and 'l'ook (at one's current location).

As a separate issue, visible in the same save: trying to 'g'et the pool of water or blood gives a message that it has been picked up with one's right hand (can be done several times), but in practice ('i'nventory or 'd'rop or 'p'ut) no water or blood is held in either hand or elsewhere.