It depends on how you define 'dark horse', really. Going with a rather loose definition, we can probably say it's something outside of the top 5 in betting odds (which are currently Sweden, Russia, Croatia, Australia and Latvia). Based on that, I do think there are a few dark horses with a chance at the win - Cyprus (34/1), Iceland (46/1 - has suddenly shortened a lot), Hungary (51/1), and maybe Ukraine (40/1). There are another few countries that I think might have a chance, but in those cases it's because they either haven't announced a song yet or they've got a revamp coming up, so there's not a whole lot of point speculating there.
Not sure I'd agree with Sweden being a dark horse - yes, they might be unpopular amongst Eurofans for once this year, but they're still first place in the odds. That's as far from being a dark horse as you can possibly get.