After seeing Gifted Aetherborn being used here, I tried it out as a 2 of on Friday (3-1) and Tuesday (2-2). Seemed good enough at those numbers without overcommiting to black.
On an iteration I want to try this week, I'm going a bit more hand attack with Tidehollow Scullers.
Following changes -
+ 2 x Tidehollow Sculler
+ 1 x Concealed Courtyard
+ 1 x Kalitas, Traitor of Ghet
- 1 x Collective Brutality
- 1 x Fetid Heath
- 1 x Field of Ruin
Sideboard:
+ 1 x Collective Brutality
- 1 x Timely Reinforcements
The Kalitas was a last minute addition (i've tinkered around with it before). But I'm worried dropping down to 23 lands might mean 3 4 drops a bit much. Just prepping this for the upcoming modern season.
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A split between cage and spellbomb is prob the best as cage also have relevant text vs green toolbox decks where it stops collected company and chord of calling I would run a 2 spellbombs and 1 cage in my 75 if I had 3 slots open.
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I have been playing BW for a long time but planing on switching to BG once I acquire my 3rd and 4th goyf. I would probaly play a list very similar to yours and here are some of my thoughts regarding your 75.
I have been running two nihil spellbomb main deck for a while in bw and it has been great. It gives you an out to all the degenerate gw-decks running around, but it also have many vs other decks such as uw-control: you can exile the grave in response to the snapcaster mage etb, you can exile the grave to delay search for azcanta, you can use it to draw a card in the event that your opponent bonces your liliana of the veil in response to the +1 ability just to name a few.
When do you side in Kalitas, Traitor of Ghet? With an extra spellbomb in the 75 I'd probably consider adding a 3rd collective brutality but that might be because I don't know your sideboard plan.
I would run an urborg, tomb of yawgmoth if I were you: with 4 colorless lands you run run a risk of color screw and urborg can help mitigate some of that risk. You currently only run 4 fetchlands and urborg allows them to tap for mana, this can be relevant in some cases when you need the fetchland later to trigger revolt for fatal push, the ability to tap for mana with out fetching (and paying 1 life) can also be relevant vs burn or if you run in that prevents you from searching or the odd case of mill when you have no fetch-able targets left.
How has Ishkanah, Grafwidow been for you so far? It feels like a card you want vs fast decks to stabilize the board but it seems quite slow at 5cmc and it also requires a bit of setup, I would probably run a second Raking Canopy as that card seems great vs both spirits and arclight phoenix and hollow one.