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>>would it be possible to use their blood as a vaccine?<<

Not exactly a vaccine, but you're on the right track.

A vaccine is made out of a killed or disabled virus. Smallpox virus is made from vaccinia, which is cowpox, but confers immunity to smallpox. Other vaccines are made from the causative virus.

What vaccines do is stimulate the immune system to make antibodies against the virus, so your immune system is ready to go when you come in contact with the virus.

A person who has already become immune to the virus has various components in their immune system which recognize the virus and kill it. The most important of these at this time is Immunoglobulin G (IgG). But IgG only works if the person donating it - in blood serum - has been exposed to the disease and has specific components of the blood which are already immune. So in a rare disease, you can't use serum or IgG from the population in general, only from those who have immunity.

So it's sort of like a vaccine, only instead of giving a killed or disabled virus, you give the part of the blood that remembers the virus and kills it.