Can I block with a previously bestowed creature?
No, as you've said in your question, all of the blockers have already been declared. There's not another opportunity to declare blockers, especially not after combat damage has been dealt!

The steps of combat are:


beginning of combat
declare attackers
declare blockers
combat damage
end of combat


Your creature presumably doesn't die until the combat damage step. You can't go back to the declare blockers step to declare a new blocker, and it wouldn't even make any sense to: by the time your blocker takes damage and dies, combat damage dealt to you by unblocked creatures has already happened too, so there's nothing left to accomplish by blocking.

And if your creature dies after blockers are declared, but still in the declare blockers step - destroyed by an instant, perhaps - it's of course still too late. If on the other hand it dies before blockers are declared, the bestowed-aura-turned-creature can certainly block.