  The universal sl_2 invariant of bottom tangles has a universality property
for the colored Jones polynomial of links. Habiro conjectured that the
universal sl_2 invariant of boundary bottom tangles takes values in certain
subalgebras of the completed tensor powers of the quantized enveloping algebra
U_h(sl_2) of the Lie algebra sl_2. In the present paper, we prove an improved
version of Habiro's conjecture. As an application, we prove a divisibility
property of the colored Jones polynomial of boundary links.
