Blackwell turned to Crossley after Steve Simonsen rejected a Leeds move in favour of Stoke City last week.
The 35-year-old Wales international was signed as cover for Edwin van der Sar when Maik Taylor moved to Birmingham.
Crossley is an excellent shot stopper and is famous for saving a Gary Lineker penalty in the 1991 FA Cup final when he was at Nottingham Forest.
He made two appearanes for The Cottagers during his first season, (2003/04), appearing between the posts for Wales in their 4-0 friendly victory over Scotland and has 7 caps for his country.