Depending on the context of your search, "key-mix" typically refers to one of the following:
TAL Keymix is a specialized cementitious primer used for high-adhesion tiling on smooth surfaces. beIN Sports 1 & Vegol TV key-mix-tr | Vegol tv bein sport 1
A Keymix node is used in software like Nuke to merge two images together using a matte. Depending on the context of your search, "key-mix"