So now I have to wonder...
How do you see Mickey and the Phantom Blot's relationship?
Aw, thank you for the question! It's a very interesting topic. To be honest, our thoughts and headcanons align with yours perfectly. But we'll try to word it differently or add onto it.
We'd say that they're a Sherlock and Moriarty, in a sense that they do have a relationship, and that relationship is animosity. The Phantom Blot has left a crazy strong impression on Mickey (by being so scary and putting him through multiple near-death experiences), which is why Mickey can't help but be on edge every time there's even a slight possibility that Blot may be involved. And then Mickey can't help but be on edge all the time, because he knows for sure that this guy is plotting something his every waking moment, and Mickey can't possibly let his guard down, because he needs to stop the Phantom Blot, because if he won't, who will?! You know?
And then, in turn, Mickey Mouse has left a strong impression on the Phantom Blot (though the latter would never want to admit it). In Blot's mind, Mickey was just an insignificant bug at the beginning, so he must've felt such a whiplash when our beloved mouse was able to stop him. And then again. And again. That's why Mickey in particular, this little nobody of a mouse, has suddenly piqued the Phantom Blot's interest (against his will).
What they feel is mutual hatred, but they also get a sort of joy out of irritating each other. Mickey is often cocky in face of the Blot for the sake of showing that he's not afraid and not taking any of the villain's trash, and it's always great to push a high and mighty calculated guy's buttons. If he gets annoyed, that means he let you get to him. If he's reacting to you, that means he's noticing you. And you're winning. That's the logic, we think.
Hold on, we have a little random dialogue we've written out that may illustrate what we mean better:
"My arch enemy, huh?" Mickey asked, a cheeky grin on his face. "So I'm your arch enemy too then? Not just a nuisance anymore?"
The Phantom Blot snickered, shaking his head as his eyes wandered aimlessly, full of irritation. When the snicker died out, the irritated eyes instantly shot daggers at Mickey.
"I hate you," he sneered, remnants of a smile still in his voice.
"The feeling's mutual." The mouse squinted with the same cockiness.
So yeah, we just really like these two as a combo. Their relationship and interactions are fun to think about and dissect. As we have said before, being sworn enemies is a sort of committed relationship. And we think both of them consider each other arch-enemies. Like, "I may have other enemies, but THIS GUY!!! He's THE enemy, I loathe him so much, I need to be the one who wins". So we guess it's hard to have that without at least an ounce of mutual respect? It's weird, it's complicated, and that's very much in our taste.