I have purposed it - The Septuagint arrangement restores the parallelism:
For I have spoken, and will not repent,
I have purposed, and will not turn back from it.
Matthew Henry
Concise Bible Commentary
The prophet had no pleasure in delivering messages of wrath. He is shown in a vision the whole land in confusion. Compared with what it was, every thing is out of order; but the ruin of the Jewish nation would not be final. Every end of our comforts is not a full end. Though the Lord may correct his people very severely, yet he will not cast them off. Ornaments and false colouring would be of no avail. No outward privileges or profession, no contrivances would prevent destruction. How wretched the state of those who are like foolish children in the concerns of their souls! Whatever we are ignorant of, may the Lord make of good understanding in the ways of godliness. As sin will find out the sinner, so sorrow will, sooner or later, find out the secure.
SDA Bible Commentary (limited)
Volume 1-7
28. For this. The elements of nature are personified and represented as mourning over the desolation of the earth.
Be black. The heavens are represented as shrouding themselves in dark clouds of mourning for the desolate earth.
Because I have spoken. The coming woe was as certain as the sinful condition that called forth the punishment.