I have two BT's with seal casts to their coats. They look completely black, even in the light, then when the sun hits them, there is a reddish tinge or glow--they are still black looking in the sun with that reddish tinge. They never look solid brown. Here's a rather rude website, but you scroll down to the bottom to see what true seal looks like:
http://www.boston-terriers.com/offcolored.htm
I don't share the exact sentiments of the author of the site as I do see some breeders breeding 'off' colors in the same way as a top notch regular breeder.