I’ve been at two wineries which offered visitors the chance to blend their own wine. You got bottles of a single variety then using a measuring tube you made blends you tasted - when you had chosen one they made up a full bottle to the proportions you’d chosen. Funnily enough I came across the label from the first of these.
A couple of years ago our U3A tasting group competed in the BBC - Blauwklippen Blending Competition. The South African winery sent a case of mixed varietal wines and using the supplied measuring tube we tried blends - we had Malbec, Merlot, Shiraz & Zinfandel there are rules, no more than 40% of a single variety and all varieties have to be present.
Different source varieties every year - you can see this years varieties and rules here https://www.blaauwklippen.com/bbc-registration/.
We came very close to the winning blends but missed out to a German wine club. But it was a hell of a lot of fun and a great learning experience.
And then when we have had several open bottles on a table we’ve experimented with adding other wine(s) to the one in a glass, on the spur, no planning, no notes. It’s fun and there are surprises!