Jessica Simpson wore an all black bridesmaids dress to BFF CaCee Cobb's wedding -- and everyone knows black is the most slimming color EVER, so .... let the baby-hiding-speculation continue!

Jessica reportedly has a tiny person growing inside of her, but has yet to confirm a pregnancy. 

But on Saturday she showed up to Zach Braff's mansion -- where the wedding was held -- in this kind-of-poofy black dress ... which CONVENIENTLY covered her stomach (aka her baby incubator).

So either she's hiding a bump ... or ya know ... just wanted to look skinny.