If an opponent, or anything else, makes me sacrifice a creature, and I have a Blood Host or something similar, can I choose to sacrifice the creature with the Blood Host?
As a general rule - No.
In response to your opponent forcing a sacrifice, you can trigger your Blood Host's ability to the top of the stack. This will then sacrifice your creature for the Blood Host, and then any other abilities remaining on the stack will resolve. If this is the only creature that you have on the table, then the original sacrifice effect will be unresolved, exactly as if you were unable to sacrifice at the time the spell was cast.
If you have multiple other creatures on the field (As would be the case with your Blood Host being on the battlefield to begin with), you would have to chose another sacrifice for the initial effect.