I know that non-transparent lumps in testicles are a problem, but can lipomas be non-transparent?

Thank you for sharing your question.  Lipomas are not transparent as they are comprised of your natural adipose tissue.  Attempting to shine a light through them will only reveal an appearance similar to any other part of your body's skin and adipose tissue.  Hope this helps.

Diaphanoscopy or transillumination will not be helpful for lipomas. Unlike a fluid filled structure, lipoma will NOT transilluminate.BestH Karamanoukian MD FACS

Lipomas are made of fat like the fat under your skin. Shining a light into a lipoma is not particularly useful as a test. Ultra sound in the hands of an experienced ultrasonologist is a good way of assessing a lump or lipoma.
A common scrotal lump is a hydrocele. These are full of fluid and light up when you shine a light into them. This is called transillumination.
I would recommend  that you to take the advise of suitably trained and experienced medical specialists. I would be less concerned that it was not transparent.
I hope this helps.
Best wishes.