Abyssal Bridges

Abyssal Bridges enters the battlefield tapped.
{t}: Add {u} or {b}.
{1}{u}{b}: Abyssal Bridges becomes a 3/2 blue and
black Abyssal creature until end of turn with: "Whenever this creature becomes blocked,
defending player mills 4 cards."
At times, it seems like the islands are helped along as
they sink beneath the surface...