> I want to insert from a Blob file into a table.

Firebird server itself (nor standard ISQL command line tool)
does not have any function for inserting file into blob directly
using just SQL.
You have to write the code yourself, of use some third party tool.

> I know, I can do this with ":h00000:00001" format, but what are these numbers for, and how can I compute it?

This is not Firebird's feature/format. You have to look into documentatioin
(or ask on appropriate group) for the specific tool you are using
(IBExpert I guess, and I think the numbers are offsets so you can
store just part of the file, but I may be wrong).

Ivan