D&C 10

As the Lord reminds the Prophet Joseph Smith of the lost manuscripts, he tells him -

"...you also lost your gift at the same time, and your mind became darkened." (Doctrine and Covenants Section 10:2)

In the early revelations in the Church, at the grassroot of preparation for the growth of the Church, the Lord seems to be teaching first the basic principles of revelation, the Holy Ghost, enlightenment, and also darkness.

Of the First Vision, the Prophet recounts -

"...I was seized upon by some power which entirely overcame me, and had such an astonishing influence over me as to bind my tongue so that I could not speak. Thick darkness gathered around me, and it seemed to me for a time as if I were doomed to sudden destruction." (Pearl of Great Price JS-History 1:15)

The reality if darkness and Satan is taught as the Lord warns him -

"...Pray always, that you may come off conqueror; yea, that you may conquer Satan, and that you may escape the hands of the servants of Satan that do uphold his work." (Doctrine and Covenants Section 10:5)

The Lord makes it clear that He will -

"...enlighten thy mind; and now I tell thee these things that thou mayest know that thou hast been enlightened by the Spirit of truth..." (Doctrine and Covenants Section 6:15)

And that He -

"...will tell you in your mind and in your heart, by the Holy Ghost, which shall come upon you and which shall dwell in your heart.
3 Now, behold, this is the spirit of revelation..." (Doctrine and Covenants Section 8:2 - 3)

And that -

"...that you must study it out in your mind; then you must ask me if it be right, and if it is right I will cause that your bosom shall burn within you; therefore, you shall feel that it is right." (Doctrine and Covenants Section 9:8)

And if it is not correct or not according to His light -

"...you shall have no such feelings, but you shall have a stupor of thought that shall cause you to forget the thing which is wrong..." (Doctrine and Covenants Section 9:9)

In my mind, stupor of thought is not necessarily darkness but the absence of additional light. But when thoughts come as an influence of Satan, that is darkness or the absence of light.

No wonder the Lord says of himself -

"I am the light which shineth in darkness, and the darkness comprehendeth it not." (Doctrine and Covenants Section 10:58)