That's odd. Presumably it's a feature their suppliers have added for journey planning purposes, to allow a search for itineraries to either of the two stations in Blackpool.
But there is no Blackpool Stations Group and hasn't been for decades.
Last I heard there was some sort of (informal?) agreement to accept tickets to Blackpool North at Blackpool South though, but I am not aware of anything in writing.
Perhaps some of the Northern Guards on this forum might work the line and be able to shed some light on this...
There are previous threads on the subject
https://www.google.com/search?client=firefox-b-d&channel=trow&q=blackpool+north+south+tickets+site:railforums.co.uk