I agree with the consensus that the Dutch aren't dark horses, but you aren't alone in your definition, Aussie! A reporter in The Observer, Amy Lawrence, wrote an article on Sunday where she named Holland as the dark horses in the headline. She followed that up by including Argentina as one of her best longshots! From a betting perspective that is nonsense. Holland are best priced 10-1, Argentina 7-1.
In my opinion I don't think you should call any of the 8 group seeded teams dark horses. They should all reach the quarters, and I include South Africa in that because FIFA always find a way (or a referee) to get the hosts to at least that stage. The winners will almost certainly be one of these seeds.
For me a dark horse is one of the other 24 teams, and the best any of them can realistically hope for is the semi-finals, so I'm looking to back a dark horse team to reach the final four.
The way the draw works means that teams in groups A to D cannot meet teams in groups E to H before the semi-finals. It is like two separate competitions until then, with groups A to D the much weaker half. If we get a longshot semi-finalist I think it will come from this half of the draw. I prefer to go with a side from Africa or the Americas, so my shortlist is:
Mexico (13/1), Uruguay (14/1), Nigeria (14.5/1), USA (10/1), Algeria (79/1), Ghana (13.5/1)