Revealing mistake: When Traci Lords approaches Blade, he tries to shoot her with the shotgun only for it to click, empty. He then proceeds to impale her with the silver stakes mounted in the end of his gun. If you look closely (probably more noticeable in full screen), you can still see the tips on the end of his stakes when she hisses at him, meaning that they're not actually inside her, even though she disintegrates in the next shot as if they were.
Continuity mistake: During the final battle, right after Frost's arm regenerates and he says "It's too late, Blade!", Blade begins swiping his sword at Frost rapidly. You can hear the sound of Blade's sword slicing into Frost's shirt, and even see numerous pieces of fabric from the shirt flying about in the air as Blade shreds off pieces. Yet, for the rest of the scene, Frost's shirt is almost completely undamaged, save for one or two small tears. This makes no sense, as we had just seen and heard Blade slicing off numerous pieces of fabric from the shirt. The pieces seen flying through the air were actually added later using computer effects to make the scene look more dramatic, thus creating the continuity problem.
Factual error: During the subway tunnel fight the subway train takes over 60 seconds to pass by Blade and Quinn. If the train is traveling at 30 miles per hour then the train would have to be a half mile long, or 66 train cars at 40ft per car. A standard subway train is only 8 or 10 cars long.
Continuity mistake: When Karen turns on the UV lamp the first time and starts to burn Pearl (the enormous record-keeper), watch closely. In the first shot from the front, Pearl has no burns yet and the air is clear. It immediately cuts to an over-the-shoulder shot and not only does Pearl's arm instantly have burns on it (that had no time to appear between cuts), but there's also smoke in the air that wasn't there in the previous shot. (And also had no time to appear between cuts).
Audio problem: Right before Blade throws his sword to retrieve his EDTA in the final battle, he leaps to his feet as Frost attempts to stomp downwards on him. As Frost's foot his the concrete ground (or Blade's shoe, as it's unclear from the shot as the impact is out-of-frame), the audio features an incredibly loud and out-of-place flesh-on-flesh punching sound effect, instead of a proper "shoe stomping concrete floor/Blade's boot" sound that should have been played.
Continuity mistake: When Whistler is giving Blade the injection, watch the vial. It starts full, then in a close up, you can see the serum level drop. Then the camera goes to a wide shot, and the fluid level is almost full and dropping, The next close up, and the serum level falls to completely empty.
Other mistake: Right before Blade confronts Frost at the end, he is shown beating up several henchmen vampires using punches and kicks, before grabbing his sword. As he grabs his sword, you see several of the henchmen vampires "burning up" in the background. However, it was earlier established that to "burn up", vampires had to be killed by traditional means. (Staked with silver, exposed to sunlight, etc.) So it makes no sense for these henchmen to burn up from only being punched/kicked. It can't be due to his sword, as he twirls it in front of them, and only after they are already burning up.
Continuity mistake: There is a scene where Blade and Frost are having a discussion sitting across from one another. The shot changes back and forth from Blade to Frost as they speak. On the table you can see a pack of cigarettes next to frost that changes position in every shot and even disappears entirely in one of the shots.
Continuity mistake: When Blade throws the darts full of EDTA into Frost at the end of the movie, they all just sit there in Frost's flesh, not pumping the blue fluid into him (this is especially evident after Blade kicks the final one into Frost's face). This is a blooper in and of itself, because as we saw earlier (as Blade fought two vampire guards and killed them with the same darts), the darts quickly pump the fluid out upon impact. What's more, all the darts are suddenly completely empty in the zoom-out shot of Frost's body bubbling and expanding from the reaction to the EDTA.
Continuity mistake: When Blade is escaping from the police, after he jumps across the building he uses a red door as a sheild. The police shoot the red door and it splinters. After the doctor pops his shoulder back in, the cops shoot the door again but there are no visible marks from the bullets before and instead of a wooden door it's now metal.