Well... *tries it himself*

WHOOPS, have to set the TEXTURE to add... not the frame! Gave you the wrong line... Make that Line 8 -- lol.. not sure why I put another blend mode on the FRAME... *fixes*

MOD makes it so the red darkens anything lighter than itself
ADD makes it so it lightens anything darker than itself.

With MOD, darkening it will make it more obvious (blank out more) -- Black will appear completely solid.
With ADD, making it lighter will make it more obvious. -- White will appear completely solid.

Err... wait.. .*links*
http://www.wowwiki.com/UITYPE_AlphaMode <-- there we go :D