Tail feathers 'black bits..?

Hi there, I'm new to chicken keeping so I apologise if this is bit of a daft question....
My Harry (large white cockerel) has what appears to be some fairly new feathers on his tail & around the base of them are these black bits??  As he's a broiler bird I understand he's not greatly agile so can't clean himself in a certain area very well so last week I gave him bit of a bath (with the help of a strong friend!).  We had a good look at them and managed to get most of them off.  They're just hard little black lumps??  We then sprayed the area with 'Purple Spray'  - he did look a bit funny all white with a purple tail!.  I had a look through the purple tonight (it's fading) and he still has some black bits..... Anyone have any ideas?  Many thanks!!