A long time ago, in a galaxy far, far away, someone asked me this. I answered it here. To cut a more detailed answer short, I said, ‘I’m pretty sure those four other wives are just bonds of political convenience meant to unite the different species of the Inhumans. A necessary gesture for their people - but Medusa and Blackagar? The others have nothing on that.'
It was an assumption, of course, albeit one that seemed reasonable enough. When Black Bolt and Medusa talked about them Black Bolt pointed out that they didn’t really know them. Given the other four Queens were all but strangers to Black Bolt and Medusa, and that Blackagar and Medusa were sweethearts from adolescence, it was quite strongly implied that they were political marriages, in name and title and little more.
I’m bringing it up again because Hickman’s been kind enough to all but confirm it. So! There we are.
Alas, all is not well in the House of Attilan, but that’s a spoilerific path to tread down another time.