If Blair had lost the war vote, then he would have been defeated by a coalition of Tories, Liberals, and Labour rebels. The latter are not going to vote out the government.
However, if Blair lost a vote on such an important issue would have been untenable. He would have had to resign as Labour leader and PM. Pretty much all other senior Labour figures (except Robin Cook) would also be on the losing side of the vote (unless some jumped to the rebels when they saw Blair was likely to lose the vote) - and therefore probably unlikely to become leader - although Brown (who kept quiet but didn't rebel) is a possibility. Most likely outcome, I think, is Robin Cook becomes Labour leader and PM.