dnd 5th – How does a warlock with Pact of the Blade seem to visually fight?

Rules do not usually dictate how something should be described in a narrative way. Usually, this is the DM's job, possibly in connection with the player. It is usually left at the table to decide how a rule / mechanics translates into history and this is the case here.

In reality, fighting with a weapon never only concerns strength, dexterity or any other statistic, even if the rules provide for it. Game mechanics do not always translate into easy storytelling.

Find a way to describe it that suits your table. Or, do not call attention to that. It's not because the mechanic uses the charisma that you have to describe it in a narrative way differently if you do not want it or if nothing suits you perfectly.