Subject Get field default value

I'm trying to get the field default value from RDB$RELATIION_FIELDS.RDB$DEFAULT_VALUE which is stored as a BLOB type 2 -BLR.

Say I have a String or an Integer set as a default value, I don't know how to read the BLOB BLR to get this value.  I know it is possible since Database Workbench displays the value.

How can I get the field default value?