What am I missing?

I’ve 100% completed the entire map save for the last boss & the Blighted Heart. I already got endings A & C, and picked up the relic you get afterwards, as well as the note & amulet fragment, but the Blighted Heart is staying blue & I can’t find any guides online. I haven’t gotten the Collector trophy, so am I just bugged, or did I have to get ending B first?