The phrase "damp sponge" refers to someone who is dull, uninteresting, or lacks energy. It suggests that the person is like a wet sponge that cannot absorb or retain anything exciting or engaging. Full definition
It is really important to use damp sponge or there is no benefit of using it. (averysweetblog.com)
Immediately after blending, I use the same damp sponge, pick up a little loose powder (with the sponge) and then press the powder on top of the concealer. (justalittleblush.com)
Then, I lightly blend the product in with a slightly damp sponge for uniform coverage and a smooth finish.» (janeiredale.com)