We've also had the exact same problems with our tomato plants, and we have no idea what's going on.
They were happy when we planted them, they began to flower, small tomato buds began appearing, and this is around the time we started noticing the plant withering.
Lea s turning black around the edges, and the entire plant going from a nice lush green, to a grey sickly dark green.
We've watched this happen to each of our tomato plants, even though they are in different pots and not near each other.
My theory is that this is nutrient based, or perhaps the soil mix we are using is not good, or perhaps the water we are using is too harsh on them? This MUST be a common problem...
Looking forward to hearing people's feedback!