Black chidori is the evidence that Sasuke has rikudo senjutsu chakra which contains natural energy.
However, bear in mind that having senjutsu doesn't mean having a sage mode. IMO this is analogous to the distinction between regular sage mode and curse mark. Yes they both allow users to infuse natural energy into their attacks, but the physical enhancement from sage mode is superior to that from curse marks. This might be the difference between simply having rikudo chakra and having RSM. Maybe the concentratioin of natural energy is different?
However, bear in mind that having senjutsu doesn't mean having a sage mode. IMO this is analogous to the distinction between regular sage mode and curse mark. Yes they both allow users to infuse natural energy into their attacks, but the physical enhancement from sage mode is superior to that from curse marks. This might be the difference between simply having rikudo chakra and having RSM. Maybe the concentratioin of natural energy is different?