Sauvignon Blanc is capable of giving wines of glass-etching acidity, with an aroma of juniper and pungent herbs. Semillon has lower acidity with nutlike aromas, which, when blended with Sauvignon Blanc, calms that acidity and adds another set of flavors to the high-end aromatics of SB. This is the classic formula for the dry white wines of Bordeaux.
This blend from Basel Cellars (73% Sauvignon Blanc, 27% Semillon) illustrates this heaven-made match.
The grapes here are from single vineyards, harvested and vinified separately. The Semillon sees a bit of oak, with regular stirring of the lees (the yeast cells left over from fermentation) which enriches the resulting wine and adds another layer of complexity to the flavors and aromas.
This rich texture of this wine might make it a tad heavy for shellfish (although I might be willing to suffer through shrimp poached with aromatic herbs) so chicken with basil would work nicely.
Here's a link to Basel Cellars.