Quote - > Quote - So only pretty people are allowed to grace our screens

In the kinds of roles Black gets, yes.  Roll your eyes at Hollywood, they make the rules.

I think she either knows someone, or is related to someone, or is married to someone, or really burns up the casting couch.

Yeah, okay.

She started with work in Australian TV, then got her "big break" in North American terms with appearances on Hercules.  Like a lot of Aussie and NZ actors, since they filmed in NZ.  She played Amazons, and being relatively tall and in good shape of course she'd be up for a role like that.   "Pitch Black" is filmed in Australia, so as local talent and with some SF/fantasy recognition for Hercules, she'd be a prime candidate.  An episode of Xena (already done work with the same production team), Beastmaster (SF/F again, filmed in Australia), Queen of the Damned (SF/F), Farscape (filmed Australia, SF/F), the rest being history.

If you look at her career it's pretty obvious what happened: she did minor roles in TV and TV movies like many other actors who never do anything otherwise, did one notable show (Hercules, in this case) that made her known for a specific genre, added some more roles that gave her notability in that genre, and since  Pitch Black and has done nothing outside science fiction/fantasy.  She's an actress who's proven able to do action-oriented SF/F roles, she's actually a good actress (she's had good reviews for her Shakespeare work on stage) so of course she's on the short list when such a role comes up.  No casting couch required.