changed the fuel filter today on fatty and was surprised to see a rather dark but definite blue diesel come out of the old filter, whats this? water comtamination , if so isn't the diesel being continuely pushed through the filter so any old diesel wouldn't build up ?

FWIW. Diesel in Italy has a blue hue to it.

Steve. 1968 Series IIA Pastel Green, nearly there project

Some companies dye their diesel supplies to identify theft. Haulage firms that have their own bunkers for example.

well ive not been to Italy and aint been stealing it from stobarts, just good old BP juice , could diesel from a previous owner still be lurking in the filter ??

It's quite possible it stains the fuel system so could be from a previous owner. It might even be that BP diesel happens to be blue. You don't know what they have in the additive packages they put in it. Can't say I often look at the colour when I fill up.

Sounds as though the filter element has been dyed blue by a previous owner using non vehicle diesel from Europe eg marine diesel from France. The dye was probably leeching out of the filter element causing your diesel to turn blueish.
Your change of filter should cure the problem.