Brush Entities Are Black

The most common reason for this issue, especially when map porting, is having the rendercolor keyvalue set to 0 0 0, which only affected the "color" rendermode in Goldsource (and only on sprites), but affects all rendermodes and entities in Source. If the problem persists try working through solution two below.

The second possible cause is a corrupted entity. 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.