Our DM and the other DM in our party distinctly recall seeing a creature called a "bloodveld" in a D&D 3.5e book. This creature is a colorless aquatic ooze that feeds on the blood of whatever it attacks, and is nearly invisible when in the water. However, nobody in our group can find it in any of their books, and all our Googling comes up with references to Runescape(!?). Just where was the statblock for this creature published?
I think you might be thinking of the Bloodbloater.
It's an aquatic ooze that seems to fit your description perfectly.
If so, it can be found in the 3e Fiend Folio under "aquatic ooze".