When I am using my Blaxx guitar pedal to tune my Tanglewood guitar, if I have either the Bass or Treble on my pickup turned up, it will not pick up the note. However, if they are both turned down, it tunes fine! Any ideas why?

It is normal. The guitar does not produce the perfect sound wave. It creates a lot of different waves that when you add them together they sound very close to a sinusoidal sound wave. Thus, if you cut bass or treble, you are removing information and the tuner is not able to get the wave approximation it requires to identify the note tuning.

There is nothing wrong with your system, just turn bass and treble both up when tuning!
Not only is that not true, it's the opposite of the problem he said he had. Cutting some treble and bass doesn't remove any real information that the tuner needs, and anyway he said he can't tune with them up, not down.

I'd check battery (pedal and guitar) and cables first, and clean out the input jack on the pedal. This is an unusual problem but it's worth checking the usual suspects first.