Not only is monopotassium phosphate the main ingredient in bloom boosters, it's also the main source of phosphate in most hydroponic formulas. (including my own hydro mixes)
It tends to lower pH because of it's low equilibrium constant, which of course means absolutely nothing. Just add potassium hydroxide to bring pH back up. (effectively giving you a combination of monoptassium phosphate KH2PO4 and dipotassium phosphate K2HPO4). Enough potassium hydroxide will turn all the monopotassium phosphate into dipotassium phosphate, which tends to raise pH.
Short answer is that monopotassium phosphate is a staple in hydro grows, and its tendency to lower pH is great. It acts as a good pH buffer in itself.