A ticket to "London terminals" entitles you to use the London Underground for free to get to the station of your choice.
Although both St Pancras and Blackfriars are part of the "London terminals" group, and there are plenty of trains that serve both, a ticket to "London terminals" is only, in fact, valid to one of the two.
(Except if it is valid on HS1)
But then again, there are people who don't realise a "London Terminals" ticket allows you to change stations at Waterloo/ Waterloo East to get to London Bridge or Charing Cross. I've known people get the Underground from Waterloo to both of those destinations, paying extra for a longer journey.
Even more, I've known people complain that Southwark Cathedral seemed an awfully long walk from the Tube station - which it is if you take the Tube to Southwark instead of London Bridge!