Can Evil deities have blackguard? Like Malar, Cyric, Bane?

Or do we have to use the wimpy champion of Torm class for them?

Backguard and Divine Champion (CoT) are two very distinct classes. Because of limitations, CoT is not available to evil characters.

Soooo-

Blackguards have to serve demons? :(

Snoteye ... two distinctly different classes...

A blackguard must have made peaceful contact with an evil outsider. She does not need to actively serve the outsider in question.

So that Blackguard could actually serve Bane, but have a cordial relationship with Graz'zt for example?

Yes.