How to produce a true breeding black rose

Both male and female parents have to have black as their recessive and dominant allele for their offspring in order to have a true breeding black rose. If it was between red and black, black would be B and red would be b. The male would have BB and the female would have BB. If you used a punnett square (look at pic) you can tell that it is true breeding by having BB, BB, BB, BB.

Hoped I helped!