After taking a look at the reason why they dropped support for loading .Xresources by default (cf. https://bugzilla.redhat.com/show_bug.cgi?id=1225384), I decided to test whether that old reason is still valid. It is not:
$ cat ~/.Xresources
UXTerm*reverseVideo: true
$ time xrdb -merge ~/.Xresources
real 0m0,018s
user 0m0,012s
sys 0m0,006s
This time spent is clearly below what a human would need to type in "xrdb -merge ~/.Xresources" manually and way below what a human is able to perceive. But complaining on this regression might belong to a different bug, I guess.
Anyway, having said all that, what's the "official" way to have the uxterm default colors *automatically* white-on-black now?