There are a few blended wines in Alsace, but not many...
Edelzwicker is the most well known of them all and each wine maker has their own private recipe for making it.
There's no set recipe and no wine maker will tell you their secrets (although they may tell you some of the ingredients).
The other blended wine in Alsace is Cremant d'Alsace (also known as Alsatian Sparkling Wine).
Alsace's version of sparkling wine rarely is made from one grape variety which is what I love about it.
And you'll have to threaten a wine maker's life before he or she would consider giving up the family recipe (and then you still might not get it).
Finally, Alsace wine with a Pinot Blanc label may be Pinot Blanc, Auxerrois or a blend of the two.
Other than those three, you rarely find blends although anything is possible in Alsace. There are no rules against blending wines.
But wine producers in Alsace traditionally prefer to bottle pure grape varieties (unlike the other wine regions in France where you will find many wine blends).
As for Chardonnay, I know that it is allowed to grow Chardonnay in Alsace, but it represents a very, very small part of the vineyard space here in Alsace.
It's mainly added to sparkling wine blends, but it's almost never the main ingredient in Cremant d'Alsace.
Viognier (from what I know about it) is a finicky grape variety that hates humidity which is not a good characteristic to have in Alsace because it isn't the driest region in France.
I would expect it to do better in drier climates like southeastern France.
I personally don't know of anyone growing it, but there are quite a lot of wine producers in Alsace, so it's always possible.
The main reason for these restrictions is quality control.
The best climate, soil, and methods of growing grapes are matched with the grape variety most suited for it.
One of the things that I love so much about Alsace wine is how each wine producer (and most of them are very small in comparison with American wineries) just lets the grape variety express itself as fully as possible.
Of course the weather and the location of the grapes have an effect...
And that means that every year and in each vineyard parcel, it tastes different.
It's so much fun tasting the exact same grape variety and experiencing how much it can vary from one bottle to the next.
I hope you will too! (whenever you are able to come for a visit)
Thanks for your question! I really enjoyed answering it...