I'm sorry for saying this without links, there are definitely articles on the subject you can look up if you're interested, and if they don't back up what I'm saying then absolutely tell me I'm wrong.
Cadaver dogs are a particular specialty of scent dogs. There are a lot of things dogs can be trained to sniff out..drugs, blood, then scent of one particular human, or cadavers (maybe other things I can't think of?).
Each dog has to be trained in its specialty by a handler. So one dog won't usually be able to do *all* these things, it's usually a tight specialization. Though I will caveat that by saying I'm talking more about blood or cadaver dogs with that tight specialization, it's usually one dog for cadaver scent, another dog for blood scent, another dog for tracking a human across spaces. I don't know anything about the drug-sniffing type of specialties, so I am not commenting on them.
Cadaver dogs do not sniff blood. They have a way to tell the scent of 'death', and studies have shown that they can pick up this scent if a body has been in situ for as little as two minutes, but much higher success rate when it goes up to ten minutes.
The scent the cadaver dogs pick up on is known as "cadaverine", and again it has nothing to do with blood.