So why are they so dark when viewed by solar observatories?
When solar telescopes like NASA's Solar Dynamics Observatory (SDO) observe the sun, they do so through filters. These filters are able to select a certain wavelength of electromagnetic radiation. Each wavelength corresponds to a certain temperature - generally higher temperatures produce radiation of shorter wavelengths. Therefore, the most powerful, hottest, solar flares may generate X-ray radiation (hence "X-class" flares), whereas the "quiet sun" (areas of the cool photosphere with little magnetic activity) generate longer wavelength radiation that we can see with our own eyes - a.k.a. visible light.
The tenuous gas in the sun's corona is very hot, so when the SDO studies the corona, it uses a filter that is able to "see" the multi-million degree plasma emitting radiation in a specific point of the extreme-ultraviolet (or EUV) part of the electromagnetic spectrum.
Therefore, should cooler plasma - such as the plasma encased in a long prominence - be observed inside the corona, it won't be emitting the same wavelength as the corona and will appear dark.
But what about the strange orb shape attached to the end of the prominence? That's actually a tunnel, or "coronal cavity," carved into the corona by the magnetic structure atop the prominence.
"When you look at it from the edge of the sun, what you actually see is a spherical object. You're actually looking down the tunnel. And this tunnel sits up top of the filament," NASA solar physicst C. Alex Young explained on his website The Sun Today.
Prominences and coronal cavities often appear just before the eruption of a coronal mass ejection (CME), and as described excellently by Young in the video below, this particular eruption was no exception:
As we view the sun with more sophisticated space telescopes, our eyes are being opened to the incredible dynamics that connect the sun's interior to its atmosphere. And as this video shows, erupting coronal cavities can trigger the formation of CMEs. CMEs are the target for much scrutiny by space weather experts as they can have a dramatic impact on modern technology in space and on Earth, so understanding these features is paramount to predicting the sun's temper tantrums.
Image credit: NASA/SDO