First of all, I like the land base. This is one of the rare budget decks I check where I don't have to write my mantra.
If you want to improve the mana base a little, Tainted
Mountain,
Fellwar Stone, Sol Ring and
Exotic Orchard would be possible upgrades.
Ashes to Ashes could be replaced with
Curtains' Call, but I'll think you want to get the damage in oder to push Greven. Another target removal I like in black is
Tragic Slip. Trigger morbid shouldn't be a that big deal, especially since it doesn't need to be your creature dying.
For the commander sections: I'll prefere auras and equipments in stead of instants, but that's a question of playstyle and your playgroup.
If you want to make use of Greven's second ability, some creatures with their own recursion effect like
Reassembling Skeleton could be useful. It may be worth it taking a look around in this direction.
In case you want to be nasty,
Maddening Imp could be interesting. The rule text is quiet complicated but gets reduced to this: "{t}: at the end of current turn, destroy all creatures this player controls, that haven't attacked". The interesting part here is, that the card doesn't
exclude creatures that have summoning sickness. The card is situational, but can be very nasty if used right. Especially against high cmc commanders without haste.
/edit: I haven't noticed the "No manarock" part
but maybe the special ban list here just includes the common staples like
Sol Ring etc.