The main reason gas logs turn black is soot buildup on the logs, which can be due to fuel burning improperly in the fireplace, or even lack of ventilation in the fireplace. Thankfully, these are two things you can fix, and you can usually prevent this from happening.
To prevent the logs in your gas fireplace from accumulating soot, you should:
Make sure the log or location is correct and meets the manufacturer's recommendations
Make sure the logs don't block the air supply or fuel supply to the fireplace
Clean any soot that has settled on the logs
Clean burner ports according to manufacturer's specifications
Make sure the air entering the fireplace is not obstructed
Check your vents or blinds
Make sure there is adequate ventilation in the room
Check your chimney or flue for soot buildup that could interfere with ventilation
If all else fails, you should have your fireplace inspected and repaired by a professional.
