I change the oiland filter in my 325i about every 6 months (when lights go out) but about 2 month after I change the oil and filter it's like black tar again looking like it need changing !

Does the engine need a flush? anyone else got this problem?

The oil will go black as it picks up crud from the heat of the combustion process & blowby gases, but it is dependant on the mileage covered and your driving style.  This is normal providing the oil consistency remains about the same. 

I've seen the oil from some engines go black after only one hour, running on the test beds at work.

By the sounds of it, you do a lot of miles if you have to change every 6 months!! 

It's done 93k, Ive only done 30k in 6 years, but I change it every six months because it goes so black.

Thats why I think it needs a flush

There are many myths about cars, one being that you need to change the oil when it goes black - this isn`t true.

I have run synthetic oil in my M3 for the last 3 years and changed it every 3k miles but even today it still goes black after about 3k miles. I no longer change it every so often because it is a waste of money, synthetic oil lasts a long time.In fact with old cars especially the filter will need changing before the oil does :).