Surrey,
United Kingdom 
why are my tomatoes going black
22 Jul, 2010
Surrey,
United Kingdom
why are my tomatoes going black
22 Jul, 2010
Probably Blossom End Rot. This is caused by an inability of the plants to distribute calcium to the fruits as they form.
There's some handy info about causes and solutions at the following RHS page:
http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=395
22 Jul, 2010