Joseph Blake and Sarah married in 1804, so I don't think that baptism is likely.

There doesn't seem to be another likely candidate in Alverstoke/Portsea area. There are some a bit further away:
Joseph son of Thomas bapt 20 Sep 1778 Lyndhurst
Joseph son of William bpt 13 Jun 1779 Eling
Joseph son of Richard & Mary Blake, bpt 1 Sep 1782 South Stoneham
Nell