I disagree, both are from outside reality,
I really don't think the corruption of warlocks is from outside reality. I think it's just a consequence of human nature and the nature of magic. We're told over and over that magic comes from who and what you are, that you have to really fundamentally believe that something
should happen before you can make it happen with magic.
So using dark magic draws on the darker aspects of a wizard's nature, and strengthens them. They couldn't do it in the first place if those darker aspects weren't there, and doing it reinforces those aspects more and more, makes the darker aspects of their nature more and more dominant.
I also think the White Council tends to overstate the degree to which warlocks 'inevitably' become corrupted.
Some clearly do, but...
Harry doesn't seem to get the same kind of dark-magic-taint-urges as he did in the early books. Maybe the Winter Mantle has just overridden that, but it wasn't really a thing in WN/SmF/TC - once Lash stopped being evil but before he took up the Mantle - either.
And Molly was using mind magic quite a bit, and her POV in Bombshells doesn't seem warped by it. She's damaged in other ways, but it seems more PTSD and grief and guilt type stuff, not "I'm going to start controlling everyone's mind for my benefit" stuff.
The way the feeling is described in book when touched upon, being greasy, cold, ect. If magic is part of creation, then it's opposing force is an element of destruction, it isn't natural to reality, has to come from somewhere.
Eh, I think there's a couple of different kinds of dark magic.
Harry in DB talks about how Cowl's magic doesn't feel like other dark magic he's encountered. He also talks in the same book about how necromancers and vampires draw from some kind of "death power" that's different from the "life power" Harry and other normal wizards use.