Why does it say "Ani Hashem v'Ein Od"?
Malbim: At this time, an Emunah spread in Bavel that there is a source of light and good, and a source of darkness and evil. They are constantly at war. In the end, light will win. Really, darkness and evil are merely lack of light 1 and good. Hashem is purely good, but He decided to create a physical world. Physical beings alternate; sometimes they lack. When man's natural strength ceases, he dies. Light dispels darkness, but when the light ceases, darkness comes. Shem Hashem is Midas ha'Rachamim and the source of all good. Elokim (Midas ha'Din) limits things, and leaves room for evil. Here, the Navi refutes those who believed that there are two sources 2 .
It says "u'Vorei Choshech" (7)! Malbim explains that it is not truly a creation (refer to 45:7:2:5)! In Egypt, "v'Yamesh Choshech" (Shemos 10:21)! Perhaps Makas Choshech was different. (PF)
Also Radak (7, citing Rav Sadya Gaon) mentions people who believed this.
What is the meaning of "A'azercha v'Lo Yedatani"?
Radak: I will put a belt on your loins before you will know Me. I already spoke via My Nevi'im that I will give to you strength over nations and Bavel.
Malbim: I gave to you strength even before you recognize Emunah in Me. I intended to publicize Emunah of My unity.
