In cooking, this term refers to a method of preparing vegetables. A large pot of water is brought to a rolling boil, and the vegetables are immersed for no more than a few minutes. Despite the name (and Webster 1913's definition), they should only lose color if overcooked. When properly blanched, green vegetables (notably broccoli) will actually look brighter. I believe this is due to chloroplast membranes rupturing and spilling out chlorophyll.

Blanched vegetables will be firm and a little plump, but less crunchy. Their flavor will improve, becoming less bitter but not stinky or sulfurous.