#possibly black as per the Slayer and the tendency of Bhaalspawn to develop black blood

[y-rhywbeth2]

'Even without using domain spells, divine spellcasters may possess magic with visual effects appropriate to their deity. For example, an inflict critical wounds spell from a Banite cleric might cause the caster’s hand to turn black and radiate beams of green light. A searing light from a Lathanderite cleric might manifest as a miniature sun that fires a beam. A hallow spell cast by a Silvanite druid might cause moss, lichen, and small plants to grow on unattended surfaces. An implosion spell cast by a cleric of Ghaunadaur might consume the target in horrible black slime.' [...] '[Divine magic will] bear the signature of the deity that created them, and allowing this signature to alter the appearance of the spell effect makes divine spellcasters of different deities unique. It also shows the hand of the deity in every spell, reminding all observers that this is divine magic in action.' - Deity Do's and Don'ts

Customising the aesthetic of spells is always fun.