Using 是 properly in sentences
The confusion here lies with the fact that「黑」(black) is an adjective, but「黑色」(black colour) is a noun. So, the correct way to say your example meaning is one of these:


她什麼衣服都很黑 (not 黑色)
她什麼衣服都是黑色的 (not 黑)


「的」should be appended on the end of the second example to make it a nominalised adjective, and「色」is optional:


  她什麼衣服都是黑的


This syntax equates the following two:


黑色的衣服
衣服是黑色的