Flames of the Blood Hand deals 4 damage to target player or planeswalker. The damage can't be prevented. If that player or that planeswalker's controller would gain life this turn, that player gains no life instead.
The damage is unpreventable, and the targeted player’s life gain that turn is replaced with nothing.
The replacement effect applies only to the target player, not to a permanent or player the damage might be redirected to.