The short answer is that it’s presumably some vim theme that he likes, but I’d guess that the origin of that is that DOS text-based applications had a long-running convention — not always universally used — of using white text on blue, unlike the Unix convention of white on black.

You can see that persisting in things like default Midnight Commander color choices (it’s set up to look like the MS-DOS Norton Commander):

…or in Network Manager’s console-menu-based utility, nmtui. I think that the dialog package and prior to that, the newt package, both for showing curses-based menu-based interfaces, also defaulted to white-on-blue, probably for the same reason.