In de MainActivity:

@Override
    public void onBlindwallAvailable(Blindwall blindwall) {
        blindwalls.add(blindwall);
        Log.i(TAG,"Blindwall added (" + blindwall.toString() + ")");
        blindwallAdapter.notifyDataSetChanged();
    }

in BlindwallAsyncTask:

private OnBlindwallAvailable listener = null;

public BlindwallAsyncTask(OnBlindwallAvailable listener){
        this.listener = listener;
    }

public interface OnBlindwallAvailable {
        void onBlindwallAvailable(Blindwall blindwall);
    }