My greatx3 grandmother was a Mary BLUNT of Eastrea, Whittlesey who is distantly related to 'your' Benjamin.
As 'anjo' has pointed out, Benjamin is very likely to be the son of William and Martha (GROUND) BLUNT and born c.1807. These weren't a line of BLUNT's that left wills, unfortunately, but by the process of elimination, the parents seem right to me.
As to William's parents, I'm not quite sure, there are two couples who could be candidates.
I understand Sophia died in 1850 and Benjamin remarried in 1852. He may have married for a third time in 1858! This information came from a contact on GenesReunited and I haven't double-checked it myself. I've got the transcripts of the local PR's, so I may well do so, now you have revived my interest, haha!