Brush Entities Are Black

The most common reason for this issue are corrupted entities. The best solution for corrupted entities is a complete replacement of them. Recreate the entity by right-clicking on the brush and selecting Move To World, then Tie to Entity under the same menu. You can also use the buttons on the New Objects toolbar. Now select the entity you had previously and try again. If the problem persists try working through solution two below.

The second possible cause, a common for map porting, is having the color keyvalue set to 0 0 0, which worked fine in Goldsource, but it shows for all rendermodes in Source.