This can be more of a problem, but can be corrected by a competent fireplace service technician. The black is the result of soot from incomplete combustion. One of these scenarios is likely occurring: 1) the air to fuel ratio is not adjusted properly, 2) the logs are not positioned exactly as the manufacturer specifies, or 3) the burner may have some clogged ports.
The most common issue is the air to fuel ratio. Not enough air equals excessive soot and blackened glass. If the fireplace is new, adjusting the air shutter is done on the initial installation setup. Air/fuel adjustments, log placement and clean burners can also be fine-tuned by your dealer during an annual tune-up and inspection.