Look, I hate to interrupt this discussion of aesthetics, but under extraction has a precise physical meaning. It means less than 20% of the solids are being dissolved in the espresso. If you want more of the coffee to dissolve, you need to either grind it finer, or use less for the same amount of water, brew time, etc. Or you can do both. Dosing higher and keeping other factors equal will always reduce the level of extraction. This is physics, not art or taste.

Whether what the OP tasted is "definitely underextraction" is another question entirely. People tend to use the term for weak shots, or shots that lack depth. Neither of these proves under-extraction. The only thing strongly indicative of under extraction is too few slow dissolving ingredients in the shot. If the blend can produce caramel, or some peaty flavors, and you are not getting these, you are underextracting. Acidic flavors, bright cutting flavors like oak or lemon peel extract quickly and are not diagnostic of either under or overextraction. I used to think all dark roast flavors dissolved slowly, but this is not true either, you can get clove and blackcurrant in underextracted shots.

In other words, it's not a trivial matter to guess the solids extraction from the taste, and I'd have low confidence in any guess I make with a blend I hadn't worked. Once I know what a low dose, fine grind, long pull shot tastes like, I know what the blend is like overextracted. Similarly, a short pull shot of a coarse grind and overloaded basket will give me the underextracted taste. Once I know these, I'll be a lot happier saying what's under and overextracted in a blend.

Chris you are probably right on that point. Unless you are willing to bake pucks or invest in a good refractometer, it may be best to just find out what low dose and high dose and high dose shots taste like, without labeling them in extraction terms.

In general, the over-extracted low dose ones will have lots of caramel and more subdued flavors, while the under-extracted higher dose ones will be less sweet but have more marked flavors both acidic and bitter based. But there's been lots of exceptions.

Here's the graph of shot time versus yield I recorded when baking pucks:

The yield axis (Y) figures are uncalibrated, add 25%.

Go 15 seconds (post dwell) to get a drinkably underextracted shot.

Getting over-extraction by lengthening the shot time is problematic. The puck/basket geometry seems to dictate the maximum yield level. For over-extracted, you are better off down dosing and grinding fine.