Blob is evil.
At least in MySQL.
MySQL stores the blobs in the table itself. So if you store a 350kByte file or text in a blob, the datafile grows by 350kByte.
If the engine needs to find data at the end of the file, it must skip through those 350kBytes to get to it, which can get very slow.
Another thing to watch out for is evil queries like
SELECT * FROM table
when you only need for ex the username.
if you have a lot of blob data then all those blobs (which can be tens megabytes in size) are selected and fetched and buffered etc, whil you really only need a few kbytes for the usernames.
In many cases it really speeds things up if you put all the blobs in a seperate table, and link it to the main table through the record id.
