Are you able to reinsert the black strip and apply a dap of glue to hold in place while a replacement is sourced?
You can buy a new Genuine roller for £42 RRP. While a complete new head would be £65 RRP. This would of course depend on the model you have. Do you know what model you have?
If you can reinsert and secure with glue it would save you quite a bit of money. If the machine is less than 2 years old, this would still be covered under the Dyson warranty, so you could ring them on 0800 298 0298 to try and get one under warranty.
The idea behind the black bristles is to stop static build up on hard floors, thus giving a better dust pick up. However some customers who have these where the black bristles have worn down have stated that they have not noticed a reduction in performance...... something to think about