"Blackness" vs "darkness"

I have been told that darkness is better to use than blackness; that blackness is not correct.

But what about this intellectual giant Carl Sagan here (0:52):

http://youtu.be/dj_MZ6i5Dr0?t=52s

He said "blackness" with the stress on that word and he certainly is aware of English.

So, my question is: why couldn't I use the word blackness instead of the word darkness if it is more logical (reference to the black color instead of vague dark which can also be dark-brown, dark-red etc.) and Carl Sagan use it too?

I am really curious about that, because when I used word "blackness" on several occasions people smiled at me and told me it is wrong.

Is it OK to use that word in the academic world and just stay away from it when dealing with casual people who speak English?

Although listed as synonyms, they are used differently. Darkness means dark or the absence of light. For example:

Light shone in the darkness.

It's unidiomatic to say light shone in the blackness, unless it's something metaphorical.

In addition, darkness means night or evil.

As darkness fell, we started packing.(night)

The forces of darkness took over the jungle. (evil, magic, etc.)

Would be strange or would convey different meaning to say:

As blackness fell, we started packing.

The forces of blackness took over the jungle.

These are not wrong, but they don't seem idiomatic to me.

Blackness in general means the opposite of white color or whiteness(as in your case). So you could say:

Black coffee, hair, etc. or the blackness of the coffee, hair, etc.

Or

Dark coffee, dark hair, etc.

• Darkness is a shortage of light. Not necessarily absence; think shadows, nighttime, movie theaters, etc.
• Blackness is either the complete absence of light, or the presence of black.

An example of the former is a pitch-black environment, with no light at all. An example of the latter is the blackness of space, where there is light coming from the stars, but it doesn't illuminate the space between, so the space is black. Black objects, black dye, etc. would also fit the latter definition.

So blackness is dark, but darkness is not necessarily black.

I can't add much to the semantic distinctions between the two words discussed above, but I can note that 'blackness' is a less frequently used word, and as such has a more poetic sound and function. 'Blackness' indicates something more stark or profound than simply 'darkness'. Although they both have the hard consonant sound, 'blackness' carries more inherent emphasis with its æk sound.