On-line Users and Authors List

I'm just curious, really.

Most of the names on the Users/Authors lists that appear on the right-hand column (my right-hand, that is) of my screen are in blue but a few are black. Most of the ones in black are people I've IM'd at some time or have IM'd me but I'm quite sure several are not. What determines which names appear in a different colour?

Geoff