Blotchiness occurs because the wood absorbs the stain inconsistently. Where the wood fibers are loosely packed, the surface soaks up the stain more than in areas where the fibers are more densely packed. The problem of blotchiness is more common when staining soft woods, such as pine, maple and birch, which are very porous, than with hardwoods, such as oak.
To produce a more consistent finish and prevent undesirable results, you can treat the wood prior to staining. To read more, check out his blog HERE.