chicken not digesting food

One of them has probably laid a little bit of womb lining, not usually too much of a problem if as you have pointed out there is no offensive odour. Common in exbatts/ older hybrids.
Massaging with oil IMO does didley squat for an impacted crop, if you think that is the problem. Check they all have grit if in an enclosed run.
The black marks could be a result of the odd peck from the other hens common if she is low down the pecking order. Have a close look and make sure they aren't wart-like, wet or crusty looking.