A couple of years ago, a blues man (Michael "Hawkeye" Herman") turned me on to Herco picks. They are nylon and the pick part is shaped like a flat pick, but they hook onto your thumb.
When you use one of these, you can flatpick with less chance of losing the pick in the middle of a lick, and you can alternate thumbpicking and flatpicking within a song.
I absolutely love the Herco picks (about $1.50 to $2.00 each), though I wish they had a heavier duty one than their current heaviest one.
I don't play blues much, but they work as well for bluegrass picking, ballad playing, and almost any picking style I do.

Genie