indexing error - key length not specified

I am trying to set an index on a field in my table, but am getting the following error: BLOB column 'ReadBy' used in key specification without a key length The fieldtype is blob, though I get the same error whether blob or text, or the medium and long versions of each. My first though, based on the error, was I needed to set a length to the field - but it won't accept/store any length I give it (because it's a variable length?) I'm using mysql on from 1and1 (hosting provider), though I don't know what version they're running .. any thoughts? As an alternative, how much less efficient would it be to do a ".LIKE '%mystring%'" versus a MATCH..?