> Yes, you would need to declare your own blob sub type e.g. -1 and filter to and from it.
> Because if you use a known sub type then you are automatically invoking "known" filters in the engine.

Blob filter is not invoked automatically. E.g. try these commands:
SET BLOB ALL;
SELECT * FROM RDB$FORMAT;

although FB has built in filter for converting from sub_type 6 to 1, in ISQL you get:
============================================
146 1 8:100
============================================
RDB$DESCRIPTOR:
?
============================================

but if you use different program that will ask for conversion you can get:
============================================
146 1 8:100
============================================
RDB$DESCRIPTOR:
0: VARYING, max length 4, sub-type 51
1: VARYING, max length 50, sub-type 307
2: TEXT, length 1, sub-type 307
3: SHORT, scale 0
4: SHORT, scale 0
============================================

Ivan