Do blunt ends have to be complementary to stick together?

Question

If no, does it mean that any two blunt ends can just bind together? What are some uses of blunt ends?

Answer 1

any 2 blunt ends can bind together, they're all compatable with each other.  The problem is that with a ligase to ligate them together, the efficiency of ligation is much lower.  Without sticky ends, the ligation you are looking for would be far less likely, that's why people tend not to use blunt end ligation.