When a homozygous white horse (WW) is bred with a homozygous black horse (BB), all offspring will be heterozygous (WB). In the context of horse color genetics, if we assume that the white coat color is represented by the allele W and the black color by the allele B, the offspring would not express a roan phenotype but would rather express the dominant color associated with one of the parents.
Typically, roan in horses occurs when a horse has a mix of colored and white hairs, but this often involves specific gene interactions that wouldn't occur from a white and a black horse pairing without additional modifiers.
Given that all offspring from this cross are WB, they will express the color corresponding to the dominant allele in the specific context (though "roan" specifically may not arise from just white and black without additional genetics). Thus, the answer to the question is:
O percent (0 percent of the offspring will be roan).