The former Pompey loanee is now a free agent after his contract was not renewed at Oakwell.
Isgrove spent the second half of the campaign on loan at Fratton Park from the Tykes.
However, he failed to make a single appearance for Kenny Jackett’s side.
The 26-year-old departed Pompey before the final game of the regular campaign against Accrington.
The Blues had the option to extend Isgrove’s deal for the unsuccessful League One play-off semi-final against Sunderland but he wasn't required.