prova così

[6.15] I want to add a BLOB field and put an index on it, but MySQL says "BLOB column '...' used in key specification without a key length".

The right way to do this, is to create the field without any indexes, then display the table structure and use the "Create an index" dialog. On this page, you will be able to choose your BLOB field, and set a size to the index, which is the condition to create an index on a BLOB field.