As people have noted, SuperBlkPastel, SuperCinny and BlkPastel x Cinny (aka 8-Ball) will give you a solid dark snake but be aware that 1) they will become more brown as they age and 2) there is a known issue with kinking and duck-billing in these superforms so if you are trying to breed for them you may end up with some imperfect animals.

SuperMahogany (aka SUMA) produces a solid dark snake as well, but again, it tends to become brown rather than black as it ages. They also frequently have a hint of a gold dorsal stripe that appears on them.

JKR made a SUMA Cinny a couple years ago that was pretty dark however I have not seen any pics of it as it matured so no idea if it maintained that darkness.

Razor x BlkPastel and Huffman x BlkPastel make an interesting superform that is fairly solidly dark with a slate grey dorsal stripe. I have not seen any mature Huffman x BlkPastel but the adult BlackRazors I saw had stayed pretty dark and the stripe had darkened as well.

SuperGHI Mojave and SuperGHI BlkPastel (or Cinny) can also give you a dark animal with a dorsal stripe though occasionally you get some side pattern markings as well.