Old Damascus barrels may, or may not, be safe to fire with black powder and reasonable charges. Only safety testing can determine that.
Like anything, not all Damascus was created equal. Some old layered steel could have corrosion between layers making it very weak and very dangerous.
Black powder and modern powder have different pressure curves. The issue is safety. Do not ever attempt to try modern powders in any Damascus barrel.
If you do try because "some guy" said it was OK, don't invite me to the funeral.