The models were already released for public use - otherwise BP wouldn't be using them either. What you should do for the models is open up the BP credits.tbl, or look up the wiki entries for the ships, and credit the original modelers and texturers. (Many of these ships have now been revamped by Esarai or Aesaar, who deserve credit too - I believe that applies on the Karuna, Narayana, Kent, Uriel, upgraded Diomedes and maybe a few others?)
For the lore and writing, please credit 'the Blue Planet team'; just a little note such as 'based on the Blue Planet mod for FreeSpace 2, blueplanet.hard-light.net' would be awesome.