Why is the Monotype Grotesque Black different from the rest of the font?

The Monotype Grotesque fonts were not designed to work as a family, originally. There were separate designs preapred in the early 20th century as metal type, usally identified by a number rather than a distinct name. Over the years — and especially once the fonts were digitized — the fonts were bundled together as a family, but not one where the memebrs are as clsoely related as they are in families that were designed as a set. It's more like a group of cousins than a family of sibliins.

It doesn't look so different to me. "Black" is a font style that is ultra-heavy and bold, designed only for use in large sizes (posters, headlines etc.)  It isn't expected to be mixed with the regular font at the same size. The exact design choices were made 100 years ago.