Blake Lively -- Total M.I.T. ... MILF-in-Training

Blake Lively doesn't need a sexy black dress for slimming effect -- 'cause even with a big ole bun in her oven ... she's a total smoke show.

The ex "Gossip Girl" flaunted her belly at a L'Oreal event in NYC last night ... creating a dilemma for all those fans who drooled over her bod long before Ryan Reynolds knocked her up about 5-6 months ago.