Sadly if you opt to purchase a blue Staffy you need to expect some sort of skin condition. It is the norm not the exception for a blue Staffy to develop some sort of dermatitis and it is pretty normal for them to have mange at some stage in their lifetime.
I would also watch out for Colour Dilution Alopecia which is common in blue Staffs because the Blue is a dilute of black, but the blue Staffs also seem more prone to it then other diluted colours in other breeds.
Canine follicular dysplasia - Wikipedia, the free encyclopedia
Unfortunately as also pointed out blue is not a healthy colour and most reputable, ethical Staff breeders will not purposely breed for the blue colour (for the reasons you are now experiencing). Most "blue" breeders will put two blue dogs together which is a huge No No. If you like the colour, next time I would aim to purchase from a breeder who does not breed for the colour but maybe breeds blacks, you may have to wait some time for a litter that has a blue but it would be worth it.
Personally I find the blue such a common colour now it is ridiculous. I much prefer the blacks, fawns and reds.
Also it would be too hard for someone on here to know if they purchased a pup from the same breeder as you. There are close to 200 breeders of Stafford's here in QLD (and a lot of them are blue breeders). You would have to let us know an approx location of their kennels and the date the litter was born.
Although this is a quite a small forum I doubt anyone here has bought from the same breeder as you.