Boston and Richmond are about 474 miles apart by road. Split the difference and you arrive near Philadelphia, the city closest to the halfway point between them. That puts roughly 5 hr 41 min of driving on the Boston side and 4 hr 20 min on the Richmond side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Philadelphia suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Philadelphia doesn't have what you're after, New York and Hartford are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Boston and Richmond.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.