The Boxer does not carry a black gene for coat color.
I've had dark reverse brindle Boxers that appeared to be black.
They have very dark brindle coats that have so many dark brindle stripes that they appear to be completely black.
It's not really black, it just has excessive dark stripes that appeared to blend together.
The sealed brindle is not a color that is any different to any other shade of brindle.
It is a term used in North America to describe a dog that has such a heavy concentration of brindle stripes so as to have the appearance of being a dark dog.
The opposite is the "light brindle" which describes a dog having very few brindle stripes.
So in between all that, there is the:
deer red brindle
charcoal gray brindle
and so on with different shades.
These simply describe the shade of fawn ground color, exactly the same as for the different shades of solid fawn boxers.
Some people use the term reverse sealed brindle or sealed reverse brindle or black boxer as sealed brindles.
They are all still the same.
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