Personally, I never put a team or some to my blacklists. Moreover, if it is about under-performance of the team playing in certain seasons. Because sometimes, there will be a change after all. A team may play badly at that time but they may take the evaluation to get better performance in the next game. In this case, we can see the differences. But if it is about always playing bad, I think I really don't like one of the football teams in my country because of the performance and also how they play sometimes only putting forward the emotion, playing bad, and supporters are often out of control 
Personally I think there are some arguments to be made about having a blacklist of teams or players, I know that the first impression is that it doesn't really make a lot of sense especially since most people lose over the long term when they make sports bets, however I think it makes sense especially when we see a lot of volatility in the performance of a particular team or player to the point it becomes impossible to predict what it is going to be their performance against a particular opponent, so it is better to avoid teams or players like that.If you see that there's no point of following or betting to players or teams that you usually follow better start to understand that it's your money that's more important, with a lots of available team or players to select you don't need to tie yourself and keep losing your bets, there are teams that really hard to predict even you completed your assessment, you still ends up messing with your money.
A good sign to sort things out and forget about following those kind of team/player.
at least for this year but sometimes I have gambled against him...
