I have been eating tri-tip all my life...and buying frequently it since I began grilling 18-20 years ago. I have wondered the same thing. I always assumed its some sort of nodule/blood clot.
I have only seen one on a handful of tri-tips that I have personally trimmed...ever. I have NEVER seen one on a trimmed tri-tip from the butcher. From the untrimmed ones, I have always cut it out and cooked the meat. However, if I bought it trimmed, I would have returned it and may take it back on my untrimmed ones from now on if I see it again.
That black stuff on the end (post cook pic) however, is something I have never seen. I would have definitely taken that back. What grade of meat are you buying?