Huh. This is actually a pretty good observation. I'd have to go through the entire manga to check if it's true or not, but I can't think of a time when Blackbeard was all bandaged up. It's a fair point, given all the damage he's taken.
I mean, it MIGHT just be as simple as Oda not wanting a big future villain to look weak, by needing to heal. It might just be to hype him up and that's it.
Or it might just tie into his Devil Fruit. If his "Darkness Fruit" is actually some sort of "Black Hole Fruit", then maybe he's just "absorbing" the damage. Letting it go "through" him instead of allowing it to affect his physical body. His Darkness Fruit isn't a normal Logia (and may in fact be another Secret God Fruit like Luffy's), and he can't go intangible or turn into shadows himself. But, maybe he can "store" the attack inside of his body so that it doesn't actually hurt him? Like Kirby eating a enemy's attack to gain it's power. Sorta?
But, you know what? Maybe it's both simpler and more complex than we've been thinking. So, get ready for a crazy, baseless theory!
What if Blackbeard's body isn't actually "real"? What if it's a illusion?
What if Blackbeard has actually been creating Shadow Clones to do his fighting for him this entire time? Yes, just like Naruto. Only probably slightly different than that. The rules are likely slightly different.
There's been a lot of theories about Blackbeard having multiple personalities. A Coward, a Conniver, and a Carouser. But, what if they aren't exactly different personalities? What if they're Shadow Clones that have all gained slightly different personalities from their slightly different experiences? Sort of like that filler episode when Naruto's Shadow Clones mutinied against him while they were stuck on a boat. (Sure, that turned out to be a dream, but forget about that.)
https://naruto.fandom.com/wiki/Revenge_of_the_Shadow_Clones
That might explain the different personalities. And even "art errors" like Blackbeard having different amounts of teeth at different times. They're all actually different people.
And it might explain how Blackbeard is able to get to so many different places so quickly. He's able to go from Amazon Lily to attack Boa Hancock, ALL the way over to Wiener Island to attack Law, all in what seems like a few weeks, max. Halfway around the world, in a few weeks. So maybe Blackbeard ISN'T actually travelling that fast. Maybe he's just sending different Shadow Clones to different places at the same time. On Amazon Lily, Blackbeard is seen with Vasco Shot and Caterina Devon (c1059). But, on Weiner Island, he's with Doc Q, Van Augur, and Burgess (c1063). So, maybe he sent a different Shadow Clone with two different groups.
And it might even explain why he doesn't take damage like a normal person. Because the person taking damage isn't actually a "person" at all, but someone made out of shadows. Blackbeard might still feel the pain the wound would cause, but since the Shadow Clone is the one who took the damage, it doesn't actually leave a permanent wound. They just don't puff away after taking damage like Naruto's Shadow Clones do. And it's possible that the reason why Blackbeard is so sensitive to pain is that he's basically feeling the pain of TWO people at the same time, anytime he takes damage. His physical body's pain, and the pain of the Shadow Clone, both at the same time.
It is odd that Blackbeard's gone THIS long without using it in battle. There's plently of times this kind of ability would have been useful. But, Blackbeard is a schemer. And a schemer always likes to have a ability hidden away for a emergency. It's possible that Blackbeard just doesn't want anyone to know about this particular ability of his yet. So he's keeping it a secret, so he can use it as a trump card later.
Now, I can't be sure this makes total sense. There might be stuff in the manga that completely disproves this. But, I do feel like it works better than it should. And while this wouldn't explain EVERYTHING about Blackbeard, it would explain a few things.