issue with custom post type feed

Note that $query->is_feed() is true for both feeds, so you’re changing both of them with the pre_get_posts hook.

I skimmed quickly through the WP_Query object for those feeds and if you want to combine these two post types, only for the main feed example.tld/feed/, you could try:

add_filter( 'pre_get_posts', function( \WP_Query $q )
{
    if ( 
            $q->is_feed() 
         && ! $q->is_archive() 
         && ! $q->is_singular() 
         && ! $q->is_comment_feed() 
    )
        $q->set( 'post_type', [ 'post', 'blood-blog' ] );       
} );

I might be missing some checks, so you might want to double check it.

ps: it would be nice to have is_feed_home() or is_main_feed() here 😉