The way that I read the Storable doc, it seemed to indicate that it stored data in UTF8. Looking at it again, it only indicates that strings are stored that way. In light of that fact, BLOB would definitely be the right choice.
It looks like SQLite has started to actually make different data types matter recently. I started using it in version 2, when it said "everything is stored as string anyway". The version 3 docs don't make any such claims, but the main differences do appear to be with non store-fetch operations.