Does Varnish darken/make thread see through like epoxy?

Only if you don't use color preserver. Neither varnish or epoxy will darken the threads if you use the color preserver.

Right. Thought that was obvious. Let me restate:
Okay: Does Varnis darken/make thread see throug like epoxy if I don't use CP?

Varnish will make the threads translucent if that is what you mean. I find it darkens and lessens the yellow look of certain colors. How much darker really is subjective, I do test wraps to check, but I find the darkening similar to what epoxy does in my experience.

In the case of white threads you can achieve clear wraps with spar varnish.

I much prefer varnish to epoxy. If you wqant "off-the-spool" color, use Varathane Crystal Clear Diamond (blue can) as color preserver.

If you use Gudebrod 822 1-part rod varnish (water clean-up), the first coat acts as a color preserver and gives you the same results as if you used Gudebrod 811 color preserver as your first coat. I discovered this by accident one time but trust me it is true. Additional coats of 822 are then used to get the wraps to your desired depth of finish. Great stuff I think.