The thing is, even on fabric WITHOUT a waterproof finish bleaching will most likely result in spotty and uneven results and by nature weakens the fabric.
Even then when conditions are perfect it's not worth trying to bleach the large majority of the time - and with the waterproof finish it will almost certainly not work out. Lightening fabric is unfortunately not something that can be really be done reliably do you either have to return the fabric if you can, live with it, or get new fabric. If you try to bleach it, you have to be absolutely okay with the fact that it might be irreparably ruined and be absolutely okay taking that chance.
Even more important is fibre content. If it's polyester it wouldn't have worked no matter what.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan