I thought the casting of Dracula was well done

Eh, I found him really bland. Didn't even seem badass since the first confrontation he has he runs the whole time and kidnaps a baby...not very badass. If he had been attacked by Blade and kicked his ass without really trying in that moment, then he would have been bad ass.

"Listen to them, the children of the night. What music they make."

He is the second worst onscreen Dracula I've ever seen and I struggle to see why anyone would think he's a "badass". While you might argue that Drake spends the entire film playing with Blade and that's the reason for his ineptitude, that doesn't change the fact that Drake doesn't do a single thing that's impressive and the way he's presented and acted make him neither interesting nor scary.

(1) He dresses like a Twilight vampire. Open-chested shirt, leather jacket, leather pants, multiple necklaces.

(2) Look at everything he actually does in the film.

-- He decapitates a vampire who was idiotic enough to lean over his grave.
-- He feeds on 5 women who were brought to him.
-- He kills two random human goths.
-- He kills John Michael Higgins.
-- In his first encounter with Blade, his first move is to push a table at him and scream "Aah!". He then runs away (wasn't Drake supposed to be hunting Blade?) and escapes by holding a baby hostage.
-- He kills three defenseless people: Patton Oswalt, a blind woman and a random black guy.
-- He remains creepily silent while approaching Hannibal and Sommerfield in the guise of Whistler (why Whistler?) but then bares his fangs and acts aggressive when trying to grab a young girl in a vent.
-- During Danica’s interrogation of Hannibal, he waits for her cue to enter and then stands silently in the background with Zoey, just like her other two lapdogs.
-- He threatens a child with death (but actually fails to intimidate her).
-- He fails to beat Blade.

(3) The scene in which he's walking down the street in slow-motion looks dumb, especially in 2015.

(It also doesn't help that this scene completely undoes the previous two scenes' attempts to make him seem formidable. First, you've got a scene with Danica and her crew that ends with Drake walking in, all smug, saying that it’s “time he joined the game”. Then, you've got a scene in which Hannibal tells Blade all about how Drake was “born perfect” and is “p*****” about his revival. So he's "joined the game", he's "p*****" off aaaaand... he spends what must be hours walking around feeling sorry for himself?)

(4) His CGI spiky form is unimpressive and, to make matters worse, we are given a pretty clear look at it in only his second scene. He also growls a lot when he's in his CGI form, which makes him seem more like a dumb animal than anything else.

(5) As you say yourself, Dominic Purcell lacks "charisma", so all of Drake's speeches are delivered in a dry, uninteresting monotone.

In conclusion, I think this Dracula is, if anything, one of the least badass villains in recent memory. The only thing that can be said about him is that he's a gracious loser. How badass.