domain name and brand guides?
 

I've come across a few domains that contain site names.

for example
a sponsor has a site called xyz.com

is it ok to buy domain xyz.net , xyz-blog.com, ect and foward visitors to xyz.com

Is this considered an ok practice?

I don't see why not - it's much the same as patenting: you're not strictly-speaking copying someone's design if you manage to change enough aspects of your replica.
:2 cents:

So if I bought something like blacks-on-blondes.org and sent you the traffic it would be cool with you or other sponsors?

just trying to make sure I understand

Well I don't actually work for DogFart (Blacks On Blonde's affiliate program), but I wouldn't see a problem with you owning that domain and redirecting the traffic under your own affiliate code, after all, at the end of the day they are still the ones who are making profit from your sales (as well as you), and receiving increased exposure because of an oversight/inability to purchase all related domains.

If you have any doubts just contact the program directly, but I wouldn't include the actual URL you want to use in the e-mail for obvious reasons. :thumbsup