I think it's likely that your James Stephen Black was illegitimate and raised by the Black sisters (whether or not part of their family hard to say at this point). I also agree that 'Oliver Black, postmaster' was likely to have existed but it's not uncommon for illegitimate children to list a made-up father when filling out the marriage register.
One thing that struck me was the death registrations for Isabella & Margaret (both on the same page) and turns out they died 2 days apart (Isabella on 23 April and Margaret on 25 April- Newtownstewart sub-district).
It's maybe not surprising that James Stephen Black turned up in Exeter by 1919. Perhaps he served in WWI and was stationed in England or had comrades from there.
Has the online tree which list his parents as Oliver Black & Margaret Curran been mentioned or explored yet?