I think the confusion is over the word "blow"--correct me if I'm wrong, but I think the OP just means that he wants to know if he can throw them away. And I think some of the responses are reading it to mean he wants to blow the insides out while preserving the shell in tact--like you might do for an easter egg you wanted to decorate and then keep. But which, of course, you wouldn't want to try and do for a rotten partially developed egg.
Is that clearer? Or did I just muddy the issue further?
And, by the way, once you know what you're looking for, you should be able to tell by Day 7 if the eggs are fertile. If I'm not sure, I wait another few days and check again. If I have one or two that are doubtful, I set them aside in the incubator (if there's room) and keep an extra watchful eye on them. I smell them every day and watch for oozing--if they start to stink or ooze, I know it's time to toss them. Also, I check every few days for development--if they've made no progress in a week, I toss them.
