Concerns about this
Note: I guess the intended use of this spell is sacrifice your own HP to save someone else, so any use that does not result in the HP running out of HP is abusive.
1. Duration of THP
Usually, temporary hit points have a finite duration. As written, the THP granted by this spell does not expire, which allows stupid tricks such as using the spell to give someone almost all your hit points, and then rest a lot to recover them. In the morning they will still have THP. Once they discover this, you will have a character that starts every day with a lot of extra HP. (There is a small amount of risk when going to sleep exhausted every night, but that is one of those risks that is easily colored as "DM is bad" if it ever appears).
2. Combining with normal spells / healing effects to create a buffer.
If someone at the party can issue to heal wounds, helpor Crom helps you, healing spirit, they can do a similar trick to turn spell slots into a THP block (and it doesn't even require a long break) The type of Blood Transfer can give you your HP, then go play at the source of healing spirit to recover everything, then (if it is fast) give your HP to another person and recover it again, and so on. And since this only requires a few minutes and a spell space and is fully repeatable, again, expect them to routinely say "we do that Blood Transfer again to get our temporary HP."
Similar problem if it is a druid with Wild Shape and can give you the HP of a body that is not even going to keep.
The only way to really fix this is for Blood Transfer to reduce maximum hit points, such as a vampire bite (which is literally a blood transfer, so there is a precedent). That sounds tough, but combined with a reasonably short duration (such as "until your next short break") really should only block spell abuse.
3. Disciple of life and blessed healer
A vital domain cleric can add 2+ (spell level) to the hit points restored by his spells. This allows them to transfer Blood for a net gain of 2 points, and since it is a cantrip, a pair of Life clerics with this spell can increase arbitrarily high in this way.
There is a similar problem with the Life domain on the 6th level, where such spells restore HP to the caster. They only get 2 HP for it, but again, it is a cantrip, they can simply send it by spam. For the rest, nothing in the spell language prevents them from choosing zero hit points and only have free healing forever.
Both gaps require you to consider that Blood Transfer is a spell that restores hit points, which may not be the case, so I suggest you discard in advance that it is not, just to avoid problems later.
4. This is not harm.
"Sacrifice hit points" is not the language that the game normally uses, but you should make it clear that the loss of the hit point no harm. It cannot be reduced by anything that reduces the damage, does not activate anything that happens when you take damage, you cannot be resistant to it, etc. For greater protection against mischief, it would be advisable to specify that the spell only grants THP equal to the actual value lost in hit points, not the number you would have lost in some hypothetical case.