The two kinds of beer have the same name, does that mean that they are identical? No, not at all. The main differenciating factor between the two are the use of wheat, their aromatization and their look.
The use of wheat
The recipes for both weiss and blanche say that some wheat should be added to the barley malt. The percentage of wheat is usually around 50%. In the case of some weiss beers, the percenage can go up to 70%, while in blanche beers the percentage of wheat is between 30 and 50% (but not more than that).
Another important difference in regards to wheat is the form in which it is used. In weiss beers, wheat is always malted, which isn’t the case for blanche beers. In some cases, some brewers add other cereals, such as oats, rye or spelt, to their blanches.
These cereals are not used in weiss beers, mainly because of the “Purity order”, which was an edict passed in Germany in 1516. Said act forbid the use of other cereals in the production of beers.
The main difference between a blanche and a weis beer is which ingredients are used in their aromatization. Blanche beers usually have coriander or orange zest as an ingredient.
Blanche beers therefore have a stronger, spicier aroma, while weiss ones are characterised by notes of banana or cloves.
Both beers look fairly similar, mainly because of their opalescence.
A clearer version of the weiss, called Kristallweizen, also exists but is brewed by very few breweries.
Both kinds of beer have abundant foam and a golden color.