Hi Chris,
thanks for your helpful guidance through this.
After trying out I realized (you marked it with very important), that the retrieval argument has to have the same name than the primary key.
In your example for the key clause id = :id this is shown.
If you name the retrieval argument e.g. 'isid', then retrieving the BLOB (with where clause id = :isid) brought me up an error. Seems not to be logical for me, but worked only in this way.
Regards
Bernhard