According to $VIMRUNTIME/colors/darkblue.vim (as of Vim 9.0.337) line 7, that colorscheme was last updated on 23 August 2022. That's quite recent, considering that that same Vim's $VIMRUNTIME/doc/help.txt (the default helpfile, as :help [with no arguments] displays it) was last changed on 13 May and that Vim 9.0 itself (i.e. Vim 9.0.0) is supposed to have been published on 28 June as shown on top of the :version screen.
Considering the above, I suppose that the colorscheme's author decided to make it "better" about a week ago and that the change ended up in the official Vim distribution. That author's name and email address are also mentioned at the top of that colorscheme source. I would have added him as Cc to the present answer (which IMO belongs in the vim_use group, as it is not specific to "only" Mac versions of Vim) except that I don't find in this Google Group webmail window how to set that. You might want to write to him yourself to know why he did it. My guess is that some highlight groups had been "forgotten" in the previous version of the colorscheme (for instance Normal set to lightgrey on black when 'background' is "dark", instead of being set to white on darkblue as "desired") and that he added those "missing" groups, defining them the way he wanted them.
IOW, I don't think the darkblue colorscheme will revert to what it was before. If you still want that, the way to get it is either to find another colorscheme (or none at all) to suit your tasted better, or to write one yourself, using as examples the various colorschemes distributed with Vim. If you decide to write your own, it will have some name ending in .vim and it should be located in the colors/ subdirectory (which doesn't exist by default but can be created) of either ~/.vim/ or $VIM/vimfiles/.
Best regards,
Tony.