Q:

How to remove black glue from Arches paper border? 

Just got an Arches block of watercolor paper, parer sheets are glues together on 4 sides with a BLACK glue.  
It seems a really good paper but i can't find a way to remove the black glue on the border once i take the piece of paper off the block. 
They could use transparent glue. 

A:

You will have the same problem with watercolor-blocks that are glued on the edges, only that you see the remains of the glue better when the glue is black.  What I usually do to remove the glue is one of the following. 

Cut the edges with a sharp scalpel (or another sharp knive) preferably with the help of a metal ruler. 
Lay the paper flat on a hard surface and scratch the edges (e.g. with your nails) until the glue is gone. 

I dont think there is a non-invasive way to remove the glue, as it is made for holding the paper to the block really tight - even when wet.