Blur Artifacts are strange orbs that make it difficult for others to spot you. The enemy can still see you, but most of their attacks will be less than accurate. Duration: Time Based
That's from the Doom ][ manual.
Anyway, in other words Doom blur equals invisibility.
And it looks like a big red eyeball... with a blue iris or whatever and then black inside that and red again in the very center. It also looks like it has a reddish-purple glare shining off of it. And I'm pretty sure it looks around or at least back and forth, left and right.
God I hope that made sense, I'm pretty tired right now :-)
Anyway, you can fire up Doom and see what it looks like.