Here's a website I found a while back that details all titles and emblems, how to get them, etc. It'll list the challenges they come from, but for most of 'em, it's up to you to find the challenge in-game. It includes the ones that you wouldn't really know otherwise (10 kills with no killstreaks equipped for luigi emblem/behind enemy lines, get 7/8/9 kill killstreaks for 6fears7, etc.

http://callofduty.wikia.com/wiki/Callsigns

Happy hunting ^.^