According to the AKC standard, the Seal Boston Terrier "appears black except it has a red cast when viewed in the sun or bright light". One way to think of this is that the hairs are black with a sable-type pattern of lighter roots that appear red when light hits them.
True seal Bostons are uncommon, however, with the increasing popularity of red boston terriers and other non-traditional colors, those colors are frequently and mistakenly labeled as "seals".
See more information about non-traditionally colored Boston Terriers
Return from Seal Boston Terrier to Boston Terrier Standards