You say that the next of kin for George's burial states only Samuel, but if you look at all the other burials on the page, usually only the father or mother is mentioned, not both.
Also you have to wonder if Elizabeth actually died before 1891 census. Although Samuel states he is a widower, this might not be true. Elizabeth might have called it a day. I can only find one Elizabeth Blackman on early census which states she was 1 on 1841 census in Portsmouth, which would still make her old enough to have married Thomas Lewis in 1862, Portsea.
Sally