a classe blob mySql tem a seguinte função:
use assim:
//(assuming you have a ResultSet named RS)
Blob blob = rs.getBlob("SomeDatabaseField");
int blobLength = (int) blob.length();
byte[] blobAsBytes = blob.getBytes(1, blobLength);
//release the blob and free up memory. (since JDBC 4.0)
blob.free();