A well-designed relational database should be normalized, and most of these examples are not because they mix different types of information in the same table. You also are forcing all attributes to be coerced to strings. Here is another approach (supposing Color and Size are attributes some blobs have and not others). You can add as many attributes as you like by adding tables. 
Blob
blob_id    INT (primary key)
blob_data  BLOB

Color
blob_id    INT (foreign key to Blob)
color_value  COLOR

Size
blob_id    INT (foreign key to Blob)
size_value FLOAT

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Searching on attributes (especially single attribute values) should actually be more efficient this way. There's a single type of index and an easy JOIN operation (which you should let the DBMS do for you instead of making two queries).

In reply to Re^3: Well Designed RDBMS by tall_man
in thread Well Designed RDBMS by nothingmuch

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