Blood of Life
If you are not undead, quaffing the Blood of Life will allow you to resurrect once, if and when you die; "Resurrect with the Blood of Life" will appear as an option on the death dialog.
If you are undead at the time of quaffing, the Blood of Life will have no effect. If you are undead at the time of death, you will not have the option to use it to resurrect.
If you have taken theprodigy, then on death you will resurrect as a Lich, even if you've quaffed the blood of life. This lichform resurrection will nullify the Blood of Life entirely (and give a message about your body rejecting it) even if you somehow become living again later.
Becoming temporarily undead via other means (such as a belt of unlife) is fine, so long as you are not undead (via a belt or any other means) when you quaff and are not undead when you die.
When playing in adventure mode, the Blood of Life will not activate until a character has depleted their adventure mode lives.
In the Infinite Dungeon, one cannot store multiple Blood of Life charges. Quaffing a second Blood of Life when a prior one is still active will give the usual message but have no effect. You must use up the first life before you can benefit from drinking another one.