Say if you had a bottle of tempranillo, and a bottle of shiraz, could you combine the two in one decanter or carafe without creating something too disgusting to drink?
Can certain wines be improved like this?

Welcome, David.

Well, as a general rule, the posher the wine, the more pure the grape and its geographical location when harvested. So therefore increasing the number of grape types and geographical locations in the mix would devalue the purity of the flavour.

It’s custom to clean the glass between wines too, so that the taste of one doesn’t interfere with the taste of the other.

A few wines are deliberately created with a mixture of grapes, but according to this site it’s against EU regulations to mix wines after they’ve been made.

However, I believe a lot of this is convention. I’m not a connoiseur - my only criterion for wine drinking is its palatability. If I found by experimentation that a really grim bottle of plonk could be enlivened by mixing it, then I’d be all for it. I have also on occasion made my own rosé with the dregs of a white and red bottle.

Blending wine. Google that phrase and you’ll find a lot more info.

A lot of blends are used, but they usually blend the grapes, cabernet franc and cabernet suvignon being the examples that mostly come to mind. However, I know an enologist (someone who actually makes a living off of studying wine) and he informs me that what really matters is that it is pleasing to YOUR tastebuds. In other words, if it’s good, it can’t be bad.