Fix Python – Find intersection of two nested lists?

I know how to get an intersection of two flat lists:
b1 = [1,2,3,4,5,9,11,15]
b2 = [4,5,6,7,8]
b3 = [val for val in b1 if val in b2]

or
def intersect(a, b):
return list(set(a) & set(b))

print intersect(b1, b2)

But when I have to find intersection for nested lists then my problems starts:
c1 = [1, 6, 7, 10, 13, 28, 32, 41, 58, 63]
c2 = [[13….