Why are my temporary veneers turning black?

How long have the temporaries been in place?  What kind of foods and drinks do you consume regularly? "Temps" do not by any means have the stability for proper sealing nor the color steadfastness of porcelain or ceramic and they are prone to stain over time or with the introduction of a highly staining compound (ie. wines, teas, coffee, tabacco etc). I would return to your dentist to evaluate STAT.Best of luck on your journey!

The temporary restorations are usually a more porous acrylic material that can pick up stains much easier than teeth or the final veneers. For this reason, it's advised to avoid consuming things that will stain the teeth (such as coffee, red wine, etc.) It is possible that the temporary restorations are leaking, causing the discoloration at the gumline in particular, for which I would recommend you follow up with your dentist so they can address that issue. Regardless, any staining on the temporaries should not be damaging to the teeth and can be removed by cleaning the teeth prior to delivering the final veneers. Hope this helps!

This could indicate that your temporaries are leaking, or your temporaries are picking up stain from something such as coffee. Likely your teeth are fine underneath. Make sure you contact  your dentist  so that he can evaluate and address your concerns.