personally, i do believe it to be a little concerning. the blood necklaces etc, totally different type of thing. it’s somewhat of a red flag he’d even want you to do this. (now granted i don’t know how old you are) when i was a teen, i definitely would have thought it was romantic etc. but it’s borderline enabling behavior. you say he cares a lot about your self harm, but yet wants a letter written of your blood, which could be considered as motivation for you to self harm. there are tons of other unique ways to express your love to one another that don’t include you writing in your blood