You'd have to define dark time. The question is maybe a bit too vague for someone to answer in a way that satisfies this sub rules.

A better phrased question would be something like "How was the life of the greek people under the Ottoman Empire?".

When considering this question, you may want to grab a gyro and a quick cup of Greek coffee, or for something lighter perhaps a plate of dolmas with tzatziki and a glass of ouzo, and reflect that all of these came about during the Ottoman period. Despite sporadic attempts to eradicate the Ottoman period from Greek history, which I thought was emblematized by the excellent Greek National History Museum in Athens essentially ignoring the era outside of revolutionary activity, it looms very large in the shape of modern Greece (I should note that every country has a selective historical memory).

But beyond delicious food, I think there are two parts to this question: was it dark for Greece (the territory of the modern nation) and was it dark for the Greeks? Greece itself was somewhat marginal during this period. This can be exaggerated: Thessaloniki/Salonica was a thriving city, Chios was then as it is now a center of Mediterranean shipping, etc. But a lot of the interior was poor and forgotten and Ottoman policy of military rule didn't help matters.

But the Greeks themselves were, of course, not just in Greece--much of the land surrounding the Aegean was, in many respects Greek. Istanbul, for example, had a large Greek population and in Izmir the Greeks formed a healthy majority. These areas were quite wealthy during the Ottoman period and Greeks, who made up a large portion of the merchants, often were the main beneficiaries. Greeks also succeeded in government: to give just one example, Ibrahim Pasha, the right hand man of Suleiman the Magnificent, was a Greek. In fact, many of the early leaders of the Greek Revolution pushed for the megali idea (Great Idea) that the Greeks must work to liberate/conquer western Turkey, and that the true capital of Greece was Constantinople, culminating in the disastrous invasion of Turkey in the wake of WWI.

So the long and short of it is that it is a complicated question depending on how you conceive of Greece.

Anecdotal but: A taxi driver in Athens told me that Greek Orthodox priests risked their lives by having secret classes in their homes, of Greek culture, and Greek orthodox Christianity, during the Ottoman times.

Can anyone give corroborative evidence of this statement?