I knwo this for sure because in the directors cut of alien there one part on disk two where they say " well why cant they just stab it with a knife ", so some one, cant remember who said well lets put something in the way of just stabbing it so thats how they came up with the acid blood.
Save for Alien3, the acidic blood has been consistently detrimental, which is why the idea for it was so innovative. Whether it came from Beowulf or Cobb, I think it's some nasty stuff.
Beowulf has been mentioned in relation to Alien, but not by anyone who worked on the film. As SiL said, it was Ron Cobb's idea to overcome a plot problem.
Over a thousand years ago (eighth century A.D.), in the first epic poem ever written in English, "Beowulf", a monster with acid-blood was first depicted. In the story, the hero Beowulf cuts a she-monster, called the Seahag, in half. The Seahag's blood is so acidic that it completely melts the blade Beowulf's sword and eats a large hole in the cave floor. The concept was rather unique for its time, don't you think? I think that the acid-blood in "Alien" owns its origins from the epic "Beowulf". What do you think?