The unfortunate thing about blush it occurs even with epoxies labeled "no-blush". No-blush epoxies probably "almost" never blush - but I've had it happen too. You have to work in a close window on recoats because blush can occur even days after the epoxy hardens. You can see it by dragging your finger on the cured epoxy.

If you're using an epoxy that's not specifically labeled "non-blush", make sure you re-coat ASAP. Or... you'll.. get more of what you encountered. Many builders let their epoxy cure a full day and still scrub the surface with a Scotch-bright pad before re-coating just to be sure. And as you did, a light sanding is also a good reassurance measure.

I don't know the epoxy system your working with in detail but I always tell people to use a non-blush epoxy for your main glassing chores. Almost all brands have them now - they started getting popular about 5 years ago.

Almost all the fast and "cold weather" hardeners blush like crazy. Slower hardeners blush way less and the no-blush "supposedly" do not.