Blood tablets are an artificial blood substitute which a vampire can take in place of drinking real blood. They are in pill form and can be swallowed directly or dissolved in a drink such as water or wine and imbibed in that way. Their flavor is apparently not very pleasant and they are not yet quite as good as real blood. Some vampires still experience cravings for blood while on the tablets.
Blood tablets are a fairly recent invention. They have been around in some form for at least ten years or so, since Kaname is seen to be taking them at the soiree after Juri and Haruka’s death. We are told, however, that the Night Class invented the current incarnation of blood tablets some years later. While it is unclear how these two facts correlate, it is possible to guess that an earlier version of the pills might first have been created just for purebloods. Since they alone are incapable of biting a human victim without also taking their humanity, it makes sense that finding an alternate substitute for them would have been a first priority. That would mean that the current and most useful formulation of the tablets which now allows them to be taken by the vampire masses, including nobility and common vampires, is an even more recent invention and was indeed created and perfected by the Vampires of the Night Class. We know that Hanabusa Aido in particular was quite involved in their testing and development.
It is implied that purebloodsand lower class vampires still take different versions of the pills, presumably formulated for their different needs. It is not known if any pureblood vampires beside Kaname Kuran regularly use the tablets or not. Acceptance for them in the noble class and below is growing slowly, due in no small part to the efforts and influence of the Night Class and their families.
One of the limitations still plauging the current version of blood tablet is that ex-humans cannot always take them. They may not work and can cause severe allergic reactions including vomiting.