There are different types (and qualities).
The green ones are really bitter, my sister loves them, but I hate them.
The black ones are either small (usually for oil) or medium and slightly blue (which is not a good thing), but if they’re completely black (or sort of brownish rather than bluish) I love them. My dad likes a variety called “hamades” which is basically soggy brown ones that are over-ripe. They’re disgusting, but somehow considered a delicassy and cost 5x more than the normal ones.
The ones you get at the supermarket are probably the generic Peloponesian ones (the ones that are not good enough for local consumption and just shipped over to foreigners who don’t know better). As with all imported goods, they’re not as fresh as if you were there yourself.
I’ve noticed a huge difference in oil too. The oil I managed to find in most supermarkets in Germany would often be over 4% acidity (usually Spanish or Italian). Greek oil tended to be slightly better (2.5%), but extremely expensive and still not that good. In Greece it costs less and is almost always under 2%. The one I use is local and has less than 0.2%. You can actually taste the difference.