Hi Molly
I'm afraid I am not expert on all this, quite the opposite.
Identity numbers started with the three letter code for the administrative area, i.e. NCA was the area code for Blackpool, as you say.
These areas were urban districts, rural districts, county boroughs, etc.
Within each of these were the enumeration districts, just like in the census. You can scroll through all the EDs on the ancestry browse facility. Blackpool has loads, from NBAA to NBPL.
Findmypast seem confused.
Article is called 1939 Register Enumeration Districts
Then says Understanding 1939 Registration Districts

I've dug out my father's National Registration Identity Card, it can't be the one originally issued.
It starts CDA, which is the code for Leyton, but in September 1939 he was evacuated to the south coast.
John