I was looking at the old Scottish folk song "I Know Where I'm Going", which contains the words:

Some say he's black, but I say he's bonnie,
The fairest of them all, my handsome, winsome Johnie

A note at the bottom of the page says that Johnie was probably a gypsy. This goes along with JA's description of the Crawfords, along with what others have said. It sounds like "black" in JA's time meant olive skin, dark eyes and dark hair, a more typically Mediterranean look.

However, the way she pairs "black" and "plain" makes me wonder if a "black" appearance was less admired in Regency times. Of course, Mary Crawford has the same colouring as her brother, but she is described as very pretty. I was just wondering if two people were equally good-looking, but one was blue-eyed, pale-skinned and blonde, while the other was dark-eyed, olive-skinned and dark-haired, would the second person be considered less attractive in JA's time just because of that?