Other mistake: Smoke rises from Karen's collar while she's lying on the table before she's injected with allium sativum. It's the shot that ends with Whistler reading her ID badge.
Other mistake: Blade talks to Karen about killing vampires using a stake, silver, sunlight or garlic and says that you aim for the head or the heart, but when he fights his mother in the temple he stabs her in the stomach with a bone and she turns to ash.
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.
Answer: I don't think it's so much a memory as it is Blade just equating Karen with his mother in that moment. The only way to convey his thought process to the audience, though, is to show his mother onscreen.
Phaneron