my tomatoes are green still, but have black on the bottom of them, can any one tell me what has happened,

This is blossom end rot. This is not a disease, but a disorder caused by lack of calcium to the plant, due to an inadequate or intermittent water supply. If the growing medium is allowed to dry out, the roots are unable to absorb nutrients effectively. A similar problem can also arise if fertiliser is added to dry soil, because the nutrients are too concentrated for absorption by the plant.
The key is to keep them moist and not wet. They also split if they have dried and then suddenly uptake water.

Mazz, I don't think the plants in the photo are tomatoes, I would say they are peppers. They may be damping off (too wet).