If you think Korrasami was out of nowhere you didn't look for clues.
1. Love is subtle. You aren't just going to be able to look at two people and say "Ya they are in love." It doesn't work that way.
2. Korra was toxic to Mako when they were dating. She always said he was taking her father's side.
3. Korra said she would much rather talk to Asami because it was easier and she said she felt like nobody would understand.
4. The blush scene. Somewhere in season 4, I think, Asami complemented Korra, and Korra blushed.
If you want more evidence I can give you more evidence. Also, it has been confirmed.