Actually I do that because It adds color, not so much to blend it
Black and white aren't really colors - they are shades, and never appear naturaly in nature so it's best to avoid using them
dark Purple/blue are good substitutes for black, and pink/offwhite etc for white of course :3
again not being a know it all
for warming or chilling? I usualy use photofilters, but thats rare :P
Im all about 50% gray
Ive actually gotten more into using photofilters lately, I think just because of that one tutorial thread that reborn posted where they were used, I kinda liked it.