A sealed brindle boxer is a purebred dog of the boxer breed that has mostly black fur. Brindle is a pattern of black stripes on red or fawn fur. When the stripes are so thick that no red is seen in between, the dog is said to be "sealed."
Boxers do not normally carry the genes for black fur. Purebred boxers only carry genes for fawn and brindle coloration, never for solid black. The last breeding line that contained true black boxers died out in the 1920s. Brindle dogs that have very heavy striping may appear to be pure black, but the genes for brindle and black are separate. Most dogs with heavy black striping are called reverse brindles. Some boxer registries prefer the term "seal" to "sealed brindle" because brindle, by the breed standard, must contain some red or fawn.