So aside from teams in Argentina or Brazil as well as not Atletico Nacional. Vote Nacional de Montevideo are easily the dark horses here. They were good last year and aside from losing Victorino they just improved their squad from last season in my opinion. It will all depend a few matches in though as it usually does in the Libertadores. On paper though I like Nacional de Montevideo. Universitario de Deportes, my team has built a very solid squad that can go far as well. Do I think we will win this? I don't. I do think though that its not impossible with the team that was built.
I don't think you could ever consider Nacional or Olimpia dark horses... they have too much history in the competition to not be expected to compete.
Agree with @locoxriver, any team that has even made it to the final four several times (let alone win the whole thing!) wouldn't even consider as a dark horse. Liga del Valle was a dark horse last year, you don't think teams will treat them with care? Once Caldas was a dark horse. So many to choose from like any of the Vzla and Bolivian clubs and even a small Argentinian club like Tucuman can be deemed 'dark horses'. Any of the top 4 from any of the top football Conmebol nations, regardless of wether they have won CL or not, cannot be deemed dark horses, imo.
Santa Fe for me as well. They won the Sudamericana last year and did well at the Copa Lib last time. Sadly not the same coach. Junior and Millos don't have good enough squads to make it far. DIM has a young good coach with a interesting Roster. But for me I think Colombia wise Nacional > Santa Fe > DIM >>>>>>>>>> Millos >Junior