When you decide to blend wines (or at least the grapes that go to make
up the wine). Do you blend the juice prior to fermentation or do you
make each separate then blend?
I know this is probably an obvious question.

Second part how important is the strain of yeast in imparting flavour to
the wine?

I currently have used a kit to make wine(s) and have used whatever is
supplied in the kit. I have started down the slippery road to looking at
various wine suppliers catalogues. (hence the second question).

I hope to experiment with the Charmat process (I have acquired a
Cornelius keg and CO2 equipment) and am looking for a suitable base wine
to try. - I have read enough (to be dangerous) and know how Champagne is
a blend of three grapes- So I am contimplating what combination to use
and of course what I can lay my hands on.

Any Guidance?

Thank you for your indulgence.