They will actually be trying to test out Einstein’s theory of relativity.  That is part of their attempt and they had this to say:

Speaking at an astronomy conference in Florida, EHT team member Feryal Ozel said all the elements were in place.

She told BBC News: “We’re almost there. The phasing in of the instruments has been done, the receivers are in place and the theoretical work has been done.

And here is an interesting video on black holes in general which is fascinating:

A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it all but impossible to observe.

thanks to NASA for the pic

thanks to bbc.com for the great info