Yes, trying to insert the blog id in an autoincrement field would cause a problem. You could add a field of type integer and add it there, though if you wanted it.
You shouldn't need quotes arount the variables; in fact, if you put them there, they won't be interpreted as variables, but as values.
If the other values are blank, but a record is created, I don't think you have an issue with the query anymore, but with no data in your variables. I'd echo them at each step until you find out where the problem lies. For example, if $user_id_from_email is empty, you're going to get nothing from there on.