Is it normal to have black soot build up on gas logs?

Soot build-up on vented gas logs is a normal part of the combustion process, and usually adds to the realism of your log set. Natural gas log sets generally produce less soot than liquid propane log sets. Sooting is not normal on a vent-free gas log set, and if soot is present, the set should be inspected by a hearth professional to verify it is operating properly.