Why Wikipedia using blob type instead of text type?
The 'right' way to store text is to use TEXT with CHARACTER SET ut8fmb4.  However, that requires the you know what encoding exists in the client, so that it can be converted (by MySQL) as it is stored.

By storing into BLOB, the above issue is eliminated.  However, that leads to another issue -- how to display the text.

Another possible excuse for BLOB is that Wikipedia was probably started before utf8mb4 was added to MySQL.

(We can only guess.)