My understanding was that the rule in the Uk has always been to replay the whole match if abandoned. In Spain I know they play the remaining time.
Some caveats to people who disagree with the decision. There had been numerous stoppages during the second half so it was likely to have had a lot of injury time. The amount of time left in the match was still a fair bit. Secondly, it was ultimately Blackburns pitch that was unable to cope with the weather. If they had good enough drainage (and I accept that it was A LOT of rain) then th a match would have been finished.