I'm tempted to say it's pronounced how it's written, but that might not be very helpful to a non-Scandiwegian. ;-)

Blomqvist, or Blomkvist, has two syllables (blom and kvist), short vowels, all letters are pronouced, the v might be closer to a u. Does that help?

(Blomkvist, like Salander, is modeled on an Astrid Lindgren character. But I think the book makes that clear.)