The story
From nigga. Exact origin uncertain, but supposedly related to or originating from the practice of members of the Bloods that avoid using words that begin with 'C', or replacing word-initial 'C' with 'B' (compare boolin' and coolin'). The substitution of the "B blood type" emoji (see "Usage notes" below) arose fittingly with the assumed "Blood" origins of the term and was perpetuated into common usage by internet meme culture.
Etymology from Wiktionary, CC BY-SA 4.0.