I have been very successful with my B/G zombies. I suggest scrapping the geists and definitely putting in dreg manglers. Imagine a turn three 3 3 with haste which also has a good scavenge ability. High yes but good. Definitely run 4 lotleths. Also I would run 2 tragic slips ( amazingg for removal) 2 abrupt decays and 4 golgari charms. Golgari charms are amazingly versatile. Destroy an enchantment, all creatures -1 -1 , or regenerate all creatures? Sickkk. Also I don't run cacklers, but to each their own. Good luck and have fun. In my experience I really have a lot of fun with this deck.