It depends what moon you’re talking about because each of the moons have a different distance from Jupiter, but I assume you mean Io, as it is in your title.
How long would eclipses last on Io?
Totality on Earth can only last up to about 7 minutes 31 seconds because of the moon’s small size, but because of Jupiter’s large size and close proximity to Io, totality on Io may last up to 2 hours.
How dark would it get?
At Jupiter, the sunlight is only 3.7% of what Earth gets. So already by itself, the sky is much darker than it would be on Earth. There’s no definite answer for this, but since Jupiter is super big, it will probably block out nearly all of the light from the sun, which is added with the fact that Io is really close to the giant, so Jupiter will appear larger than it would appear for example, Europa.
How many stars would you see?
Io also has nearly no atmosphere, only an extremely thin layer of sulfur dioxide. This means that the sky would be similar to the vastness of space, with many stars visible. Jupiter however, will cover a considerable amount of space in the sky. There would also be Jupiter’s other moons visible too. If at the right time, Saturn will appear bright in the sky, with Uranus in the background, visible to the unaided eye, but very dim.
Would you still be able to see the sun’s corona?
No, most likely not. The corona from the sun only radiates about five million miles out, and combined with the sun’s tiny size from Io and Jupiter’s gargantuan size puts the corona well behind Jupiter during the eclipse.
Of course, we are talking about an eclipse where Jupiter perfectly occulates the sun, and not some partial eclipse from Jupiter.