I'll suggest Gothitelle
I realize your current team does not has a Psychic type, which could get rid of poison type that non of your team has type advantage at.
Gothitelle has nice special attack, defense and special defense stat, and access to a not bad movepool.
It's two ability was also pretty nice.
Moveset suggestion for it:
Gothitelle @ Mind Plate /Leftovers
Suggest Ability: Frisk or Shadow Tag were all ok
Suggest Nature: Modest Nature
IVs: 0 Atk
- Energy Ball
- Shadow Ball
Hope I helps and good luck on your rest advanture in Black!