Not sure what you mean by both his parents are listed as BLACKMORE - the birth registration clearly states Albert's surname as SHEPHERD and his mother's maiden name as SHEPHERD (father's surname is not given in the GRO Birth index - it is the child's surname and then (post 1912) the mother's maiden surname.), which, as Linda V correctly stated usually means he was illegitimate. Another birth was registered in Tiverton district Sept qtr 1924 for a William R SHEPHERD, mother's maiden name also SHEPHERD.
A marriage registration is shown June qtr 1906, Plympton St Mary district with the 2 possible grooms being George HENRY SHEPHEARD & William LEE and brides Caroline Maud SHEPHEARD & Harriet Elizabeth INSKIP. I checked the 1911 census and George Henry married Harriett INSKIP, so that is not the solution.