When I start or end going around a block. I start with pull up my bobbin thread from the back, sew what I need to, then stop and bring up bobbin thread. When I'm done with the top I go back and tie tiny little knots and clip threads.
That way no backstitches showing on the back.
As far as the color of thread, it all depends on what you want. You could match your thread to the piece color of the side that you're SITD'ing. Use an accent color, I can see a black would look cool.