BLM cannot be controlled by normal means. They are an anarchist movement.
It seems like it would be a good optic but it can go badly quickly. The Democrats wants them to take over the podium. Make a few statements, start a few chants and then go away.
But they can’t control that.
Add that the Philly police may not be interested in stopping them
Despite all the warnings about Cleveland, fact is the blacks there are pretty apathetic. So the rabble rousers are mainly the paid protestors. No grass roots going on there. (It’s not over yet, though, bad stuff could still happen - hope not.) Also, it’s fairly isolated from large metro areas.
Philly might be a different story. Philly is within a few hours commute of the I-95 corridor from D.C. to NYC, so it’s accessible to millions and millions of disgruntled people. The paid protestors will just travel the six hours from Cleveland to join in, but there could be a sizable regional crowd as well.