Hi. I think he married Rebecca Jones. Rebecca Blagg is on the 1911 England Census - she has been married for 5 years and has 2 children living with her - Marjorie and Muriel. They are living at 15 Elfet St. Birkenhead. Fred was obviously somewhere else.
On the probate will records there is a record for - Blagg Frederick otherwise Fred of 63 Prenton Rd. East. Birkenhead - died 18 Jan. 1940 - at the Cheshire County Hospital Upton Cheshire. Administration with Will Liverpool 10April to Rebecca Blagg Widow effects £1533.11s
Hope this helps