I don't really see why this is an issue. In practically every Marvel movie and TV show we learn about some big new aspect of the universe we never saw signs of before, so vampires really wouldn't be any different. Vampires secretly living among humans has also been a huge part of Vampire fiction going back as least as far as Dracula, so really makes perfect sense that the same thing would be going on in the MCU.
One, as Easter Egg and tease-happy as the MCU has been with other concepts (infinity stones for one example), this film series at least has precedent where introducing an idea that was supposed to have some impact on that world. One would imagine the undead would fall into such an impactful category.
Two, Marvel's Dracula was never fully integrated with the rest of the comic MU, but he--and his "flock" were not hidden. Further, there's quite a number of vampire fiction where the creatures ultimately did not care about humans knowing they existed, because in their minds, humans were no better than powerless livestock. It did not mean they wanted to be captured (see: 1972's
The Night Stalker, or 1974's
Bram Stoker's Dracula for examples of that kind of vampire), but when it was time for business, they were not shy, nor did they care if they were seen.
I don't see why, we haven't seen Blade or any references to him before him until The Eternals, so if the vampires have been living in secret, then he probably has been too.
That makes little sense. Blade has vampiric traits, but
he never needed to hide from society or any potential threats. Rewriting the most known detail about him all to handwave the MCU never properly planting the seeds of vampires at all is lazy.
In most stories where vampires are living in secret, they have specific rules about how they operate, so that's probably the route they'll go with the MCU.
The point is that if the film follows the comic origin, vampires could not have been living in secret to the degree suggested by a few, since it was a vampire (Frost) who (acting as a doctor) killed his mother / gave him the traits of a vampire, and Frost himself had encounters with the creatures
before his own trnsformation. In that comic-book continuity, vampires were active, and not so much a version of the myth-no one-believed-until-it-was-too-late trope.