For the second part of this adventure, since they met Mind Flayers, I'm thinking it would be cool for the characters would go in space with a spelljammer ship or via a portal. I've unearthed my copy Shadow of the Spider Moon Polyhedron magazine. Do you have any suggestion as to why they would go to the moons in the context of a Blackmoor campaign ?

Perhaps you don't really need a reason for going in space! :P
The characters find a space/spelljammer ship fallen to the ground who knows when, clear it from an array of Blackmoorian monsters that crawled into the ship, some offshoot of whatever off-world mosnters they carried and perhaps some original crewman (undead?), start studying the ship and, suddenly, they find that the landscape has plenty of stars and planets... ;)


In alternative you coould make up a plot linked to the Sahuagin: perhaps something is getting wrong with the black stone of Blackmoor and the race that knows more than anybody about it are the Sahuagin. Unfortunately, in Blackmoor there are no more Sahuagin and the only ones known surviving are on the moon...