I'm Sergei Tokmakov, California attorney (Bar #279869). Quick framework on AI-generated commercial use:
(1) Output ownership: USCO has held that pure AI output without human creative selection/arrangement isn't subject to copyright. Practical effect — your concept boards are essentially in the public domain unless you can show meaningful human authorship in the curation/composition. (2) Generator ToS: Midjourney's commercial license rights flow to paid users; check the specific tier you used. (3) Training data risk: the bigger concern is whether the generated image inadvertently reproduces a specific copyrighted work — face/character resemblance, distinctive style. If it does, you could be liable downstream.
For client-facing work I recommend (a) a license/disclosure clause in your SOW noting use of AI tools, (b) for the actual logo (which you said you drew from scratch in Illustrator), document the human authorship trail, and (c) any "inspiration" boards should be clearly labeled as references rather than deliverables. Informational only.