As amateur winemakers we’re as optimistic as ice fishermen. We buy the best fruit (or juice, or kits,) use our best techniques and equipment, and expect our Cabs to smell like black currants, our Sauvignon Blanc like grapefruit, our Merlots jammy, and our Nero d’Avola dark as sin. But all babies aren’t Gerber babies and all batches of wine have strengths and weaknesses.
You can make corrections through blending. It’s OK. But I suggest you only blend finished wine and not mix varieties together before you know how they will turn out. It’s tempting to look at two five-gallon carboys of different varieties and want to siphon them into that empty 10-gallon barrel. It would be as risky as mixing everything in your fridge together and hoping for the best dining outcome.
Most commercial wines are blends of different vineyards with different characteristics. Different varieties make up Red Bordeaux wines typically with specific varieties.
Old World winemakers sometimes make a “field blend” out of several varieties in the same vineyard. Field blends can be responsible for wide variations in different years. In my opinion, you would need fortunate timing in order to have phenolic ripeness in all the varieties happening simultaneously to include several varieties in one harvest event. It is much more logical to harvest when the grapes tell you to, make the best varietal wine you can, and then figure out what each finished variety can bring to the party....