Turning to Robert George Black, the following is some very speculative stuff on a possible lead to his background, the various bits and pieces might turn out to be co-incidental and not relevant at all (mostly common names, after all), but it might be useful to pursue further if just to eliminate them. The hook for me was a 'Marshall' connection...
1. A Robert George Black enlisted at Armagh in the Royal Irish Fusiliers in December 1890 (he was already in the Royal Irish Rifles) and was discharged again in 1892, he was recorded as a single man, a labourer, born in Shankill parish Belfast, and as 18 years 10 months old. The next of kin were given as his elder brother William and mother Mary Ann [no surname mentioned] of
93 Hornby Street Belfast.
2. The 1890 Belfast street directory on the Lennon Wylie website shows the following entry for Hornby Street:
93. Marshall, Wm., drillerhttp://www.lennonwylie.co.uk/hijkcomplete1890.htm3. A 5 month old John Marshall died on 5 November 1889 at
93 Hornby Street, his father recorded as William John Marshall of that address:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1889/06127/4749985.pdf4. It seems that John above was one of twins, John and Lizzie, who were born to parents William John Marshall and
Mary Ann Black on 16 May 1889, the other twin Lizzie died aged 9 in 1898:
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1889/02477/1919560.pdfhttps://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1898/05833/4651803.pdf5. There was also a son Frederick John born to this couple in 1887 who died the same year:
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1887/02552/1944200.pdf https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1887/06199/4773032.pdf6. Someone called Mary Ann Marshall, wife of William John Marshall (a driller), died in December 1900 at
10 Harold Street with a reported age of 41:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1900/05747/4623017.pdf 7. Then there was a marriage weeks later on 4 February 1901 of a 44 year old widower called William John Marshall of Silvio Street to an Annie (Officer) McLaren of
10 Harold Street, William John was described as a driller and his father was recorded as a labourer with the same full name, the couple appeared in Silvio Street a few weeks later in the 1901 census:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1901/10296/5747831.pdfhttp://www.census.nationalarchives.ie/pages/1901/Antrim/Shankhill/Silvio/949710/8. A William John Marshall of Silvio Street died the following year (1902):
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1902/05705/4609236.pdf9. I can't readily see a marriage of Mary Ann Black to William John Marshall, but there was a widow Margaret Black who married a William John Marshall in 1881, in the marriage record, Margaret was described as 'Black alias Black', daughter of William Black a soldier - BTW, in the marriage registered directly below theirs, the couple were also witnesses on the same day in the same church to a Samuel Stewart (who was a witness to their marriage):
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1881/11015/8027554.pdfSo how, if at all, does the Robert George Black at (1) above, born in the early 1870s, fit into this story? Was the widow Margaret Black who married William John Marshall in 1881 the same person as Mary Ann Black mentioned (above) later and was this Robert George's mother? I can't yet see a previous marriage for the widow Margaret Black of the 1881 marriage, let alone one to a soldier called Robert Black (ie. which would satisfy the groom's father's details in the 1895 marriage of Robert George Black to Elizabeth Black mentioned in the original post). There was a couple called Robert Black and Margaret Ewart who got married in 1870 and had a number of children (including a son William, but no son Robert that I can see) - while Robert, husband of Margaret Ewart, did indeed die just some months before Margaret 'Black alias Black' married William John Marshall in 1881, Margaret's father in the 1870 marriage was given as Francis Ewart, which is far removed from the given father 'William Black' in the 1881 marriage. And of course, for this to be the same person who died as Mary Ann Marshall in 1900, her maiden surname should also be Black to be consistent with the maiden surname shown in the birth registrations at (4) and (5) above, not Ewart. So not hanging together as a story well at all.
Could Robert George Black have been 'adopted' by William John Marshall? Could he even have been illegitimate? Or is all of this just a red herring and unconnected to the OP's Robert George Black? Maybe someone else with fresh eyes coming to this might see something more quickly to rule this in or out.