K.H.
From googling "antibiotics and black tongue"...it sounds normal and is something that will subside once stopping the antibiotics.
Note in this link, it states black tongue is a side-effect of Amoxicillin:
http://www.drugs.com/amoxicillin.html
http://www.helium.com/items/681246-black-tongue-common-ca...
http://www.ivillage.com/what-are-causes-black-tongue/4-n-...
http://www.ehow.com/facts_###-###-####_black-tongue-sympt...