It is possible he married an Isabella SCOTT and they had a son William b. abt Sep 1866 in Campbeltown.
Duncan Blue married Isabella Scott in Campbeltown in 1860. Go to
www.scotlandspeople.gov.uk, invest in some credits and use some of them to download a copy of the marriage certificate. It will tell you the names of the couple's parents including their mothers' maiden surnames.
There were two children of this marriage; Mary, born 18 August 1862 and William, born 3 September 1866.
In 1861, at Drumore, Campbeltown are Duncan Blue, 28; wife Isabella Blue, 26; mother-in-law Mary Scot, 55; and Isabella Tod, 2.
In 1871, at Kirk Street, Campbeltown, are Duncan Blue, 39; wife Isabella, 35; daughter Mary, 8; son William, 5 and mother-in-law Mary Scott, 65.
ev has already posted an extract from the 1881 census.
In 1891 there are Duncan Blue, 56; wife Isabella, 50; daughter Mary, 25; son William, 23.
There's a death of Duncan Blue, 62, in Campbeltown in 1892, mother's surname Mckinlay.
There is a baptism of Duncan, son of Duncan Blue and Isabella McKinlay, in Campbeltown in 1824.
So I don't think that it was your Duncan Blue who married Isabella Scott, but the marriage certificate will nail that one way or the other.
It would not surprise me if Duncan Blue, husband of Isabella McKinlay, and William Blue, husband of Susanna Galbraith, were the sons of William Blue and Anne/Agnes McNaught(on), baptised in 1806 and 1802 respectively.
A look at the original baptisms might possibly clarify that.
There's also a death of Florence Blue or Watson, aged 82, in Glasgow in 1883. In the 1881 census she is listed as aged 80, born Campbeltown. If I'm right she would be your Duncan's aunt.