So the interesting thing about Mercury’s semblance isn’t that his father stole it but why. 

I’m sorry you didn’t have a mommy that loved you, but I had a father who hated me. He never went easy on me. Every day of training was a beating. And when I unlocked my semblance, he stole it with his. “This is a crutch. This makes you weak.” He told me I could have it back when I was strong. So I got strong. But I never got it back. I’ve had to work harder than anyone to get where I am. You may not like it here without Cinder, but I think I’m right where I’m supposed to be.  

There are only two reasons why Marcus Black would take Mercury’s semblance away from him and refer to it as a crutch that makes him weak. The less interesting one is that Mercury just relied too much on his semblance during combat and Marcus wanted to train him to be as strong as possible without using his semblance first. To me that seems less likely, there would have been easier ways and if Mercury’s semblance had been useful in combat, it would have made more sense to train that semblance as well. 

The second option is that Mercury’s semblance was something that wasn’t really useful in combat, or (at least in Marcus’ eyes) actually hindering from being a successful assassin. It would make sense for it to be something that has to do with emotions or empathy, because being empathetic would most likely keep Mercury from killing them, but there’s a variety of semblances that would work here. If he really had a soft, non-assassin semblance, then it makes sense why Marcus took it from him, and it also makes things a hundred times more interesting all of a sudden. 

Ren said that some think a semblance is an expression of someone’s personality. Mercury having a soft semblance (I’ll just use that term from now on, you get what I mean) would then maybe hint on him being different than his father, or on him having a soft side to him that has been buried deep under years of violence, pain and abuse. But what I think is even more interesting is that Mercury wanted his semblance back. 

It’s quite symbolic. Mercury was abused and beaten, then he unlocks his semblance and it reveals that he’s not the cold assassin his father wants him to be, that there is still a soft side to him, and still hope. His father takes that away from him, a symbol of him taking away his son’s innocence (Overrun by the hate and the beatings defiled by a father) but instead of just giving in, Mercury works hard to get strong so he can get it back, still clinging to that small part of him that distinguished him from his father. But he only ever knew pain and violence as Tyrian has pointed out, so when he tried to get his semblance back from his father violently, his father died and his semblance died with him. 

The interesting part about Mercury having a soft semblance isn’t that he had it in the first place, but that he wanted it back, that he fought hard to get it back, that he failed to get it back because he was only using the violence his father had taught him, and that he’s still bitter about his father having stolen it from him. If he really was the cold assassin his father wanted him to be then he would have already realised that his semblance really did make him weak, he’d be glad to be rid of it, but he’s bitter about being robbed of that part of him, and that shows that there’s more to Mercury than what he’s trying to show here. Tyrian has seen right through.

After all Mercury is nothing but a kid tricked into following Cinder, a victim of abuse without any drive or any direction, and all he has ever learned was pain and violence, and now he’s too afraid to leave it.