Stampin' Up!'s product guide that you use Basic Brown for stamping your line image to watercolor inside with reinkers and blender pens on SU! Cardstocks. But they suggest Brown StazOn for the stamping of your line images on Watercolor or textured papers, and if you plan to use water brushes or Aqua pens... as you would with watercolor crayons.... so stazOn for the "wet" stuff.
I use Brown Stazon for all the very "wet" watercoloring I do... If I am just using blender pens and watercolor pencils or reinkers I use Basic Brown and I have never had a bleeding or blotch problem. I would first try to re-ink the pad. I find that my Basic Brown and black need to be reinked more often than my other pads.
If you are storing the ink correctly; flat and in a room temperature location with the top side up (with the print on the top of the pad), and you are using the product on SU! card stock as it was intended to be used... and if you are STILL having those problems; then you should call SU!
It is not unheard of for there to be a "bad batch" of ink or something... SU! will make it right!