Just throwing it out there... You might want to check out nesdev for this kind of stuff.
That said, if you've checked the main input capacitor and you're still getting a black screen on startup, you might want to look at the (main PCB) SRAM chips, mainly the one for the CPU. If swapping the PPU and CPU SRAMs around doesn't change anything, then it might be the extended traces after all. Throwing some buffers into the mix might help a bit.
edit: yes sorry, I was confusing the NES and the SNES (the NES RAM chips are identical, not the SNES). I would still try the input cap though.