I'm pretty sure you can't auto_increment a BLOB. In fact, I don't think you can make a BLOB a primary key either. I'm not even sure why you'd want to. There's no way that could perform well.
I'd recommend you rethink your database design. I can guarantee you that you don't really need a LONGBLOB for your primary key. If an INT isn't long enough, then a BIGINT UNSIGNED almost certainly will be. And if not, you can fit a GUID, which has 2^128 unique values, comfortably into a VARCHAR.
Think of it this way - a LONGBLOB can store 2^32 types of data. That's 4GB. If you need that much data to uniquely identify a row in your database, something is very, very wrong.