“I have noticed that while they are sucking up dark, if something blocks them, they leave a shadow of dark behind.”
Of course. The darkness would like to, but cannot, get by the object.
You will notice that when the light bulb is switched off, the darkness returns.
The bulb does not emit light, it sucks up dark.
If the bulb was emitting light,
then when you switch it off, where does the light go?
They do not emit light, they suck up dark.
That is why when you turn them off, the darkness returns.