I have been blanking my crush lately because I hate him but also still like him. Now he is really upset and failing. How do I apologise?

To be honest, blanking on him, I'm guessing means ignoring him? So you are feeling mixed emotions towards him, ' hate and also like?.  Firstly I feel to be fair on him you need to think about your actual feelings for him. Did you develop the crush when you hated him? Or was it vice versa. Once you get this fact straight personally think to yourself,  do I want any sort of relationship? Whether friendship or even relationship? Even if you don't really want a relationship I guess it'd be very nice of you, if you did go talk to him! 

He's probably feeling bad because obviously he can tell you're ' blanking him' so I suggest you clear this up with him . From what you have asked  its not your fault that he is ' upset and failing ' but I guess he would like an explanation on what's going on :) hope that helps ! That's what I would do ! You don't need to talk to him in real life , maybe message him?