Marvel's ultimate vampire hunter is undeniably Blade, and he knows the true reason vampires don't have a reflection. Some careers don't come with a particularly good life expectancy. That's especially true in comics, where there are people who have genuinely chosen to commit their lives to the profession of hunting vampires. Such men and women naturally find themselves in very dangerous positions with alarming frequency. Given just how many powerful vampires are in Marvel Comics, vampire hunting ranks among the most dangerous career options.
The best vampire hunters tend to be skilled warriors and combatants - with Blade at the top of the list. Getting his powers from Morbius, Blade is a dhampir, a human-vampire hybrid who possesses all of a vampire's strengths and none of their weaknesses. Although Blade can use advanced technology when he wishes, he prefers simple solutions. The 2006 Blade series reveals his greatest secret, though; he's incredibly observant. Blade has learned to watch his surroundings with care, looking for anything that doesn't belong - and, particularly, the absence of something that should be there. His eyes instinctively turn to every mirror, window, and even body of water, checking to see if everyone around him has a reflection.
In Blade #1 by Marc Guggenheim and Howard Chaykin, Blade explains that in Marvel Comics, as in Bram Stoker's Dracula, vampires do not cast a reflection. According to Blade, there's a simple reason for this; "The vampyr have no soul, and those who have no soul... cast no reflection." Interestingly, that particular series establishes that Blade himself does not have a soul - making him immune to certain magical attacks that strike at the soul - and this likely means he too does not have a reflection.
This idea is rooted in real-world legends. Many historical cultures believed the mirror did not reflect the true appearance, but rather the soul; consequently, vampires, demons, zombies, and other undead are often assumed to have no reflection. The idea was popularized by Stoker, but it's actually far older, and it's interesting to see Marvel Comics acknowledge this deeper historical context. Interestingly, some Chinese mythology took a different approach, with vampiric creatures actually afraid of mirrors because the lack of a reflection served to remind them of their soulless nature.
In Marvel, vampires like Dracula originate from the Darkhold - a book of ancient black magic left behind by the Elder God named Chthon. This is believed to be the source of most of Marvel's supernatural creatures, including werewolves. It's quite possible that most of the Chthonic creatures are similarly soulless, and consequently fail to cast a reflection. Blade's vampire-hunting technique may point him to a far greater range of demons and monsters as well.