When you get down to the brass tacks of it, BlkPastel and Cinny are fundamentally the same mutation so yes, there is the same risk of complications with the 8-ball combo as there is with the single-gene supers
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Blackhead, Mahogany, and GHI are all good candidates for making darker snakes. The supers of any of them when combined with Cinny should get you close to a more or less uniform all-black (juvenile) snake. Triple combos of them will also probably get you pretty close.
There are also some morphs that may not be dramatic for a two-gene combo but can push you in the right direction if done as a triple. Mojave would be a good example. A Cinny Mojave is not particularly dark but a Cinny GHI Mojave is. Leopard and (though outside of your current price range) Acid would be other candidate morphs