Ooooh, an excellent question and honestly, I never gave it any thought 😂
I reckon that Ranpo's blood is on the sweeter side (man lives off sweet foods, I'm surprised his blood isn't sugar) but not sickly sweet like if it were just straight sugar. Sweet like fruit, accompanied with the taste of whatever Ranpo's eaten that day.
I also like to think that Bram knows this and just casually slides Ranpo food items that he wants to taste within his blood, and while he thinks Ranpo doesn't know he does this, Ranpo absolutely does and eats them anyway cause he loves it when Bram feeds off him and wants to keep that for himself 🥰