Are you talking about the black areas that almost make it look like the mantid has pupils and is looking at you? My understanding is that it is an optical illusion created when you're looking dirctly down a cluster of rods that make up the mantids eyes. I'm probably not explaining it well enough, but I'm really tired!
Their compound eyes are like a cluster of normal eyes, you can see a reflection in it. When you look from any angle you will catch the reflection in one of the sections... So it looks like s/he is looking at you, but really, it is just a reflection of yourself.
Or, if the spot doesn't move around as you do, s/he has rubbed his/her eye accidentally shortly after moulting, and it should go away after the next moult.