Hey Ed,

Just don't do it!  Nice dark, rich walnut will lighten over time. Any 
stain blending of the sap wood you might attempt when the wood is fresh 
will not lighten the same way as the heart wood.  You saw what happened 
to my work bench after exposure to sun for 10 years.  I save any walnut 
with sap wood for inside skirts or drawer pieces where they won't show 
so much.

Bill W.
In Beautiful downtown Nottingham, PA