I have Blood of the First sustained the whole time for Merrill, which compels her to use her health to cast spells. Would a trinket that gives mana/stamina regeneration help regenerate her health as it's being spent casting spells?
An item with mana/stamina regeneration would continue to operate as intended (regenerating mana or stamina), regardless of what pool you use to cast spells.
If you wanted to gain an advantage in casting spells as a blood mage, you would need to find and use an item which grants health regeneration.
so a direct and simple answer is no.
as blood magic taps merril life rather than the conventional mana. alsoi think merril can't be conventionally healed so health regenaration is not actually recommended, but items/trinkets that povide health would.