The minor illusion spell uses the following rules in the last paragraph of the spell description for how a creature determines whether or not the illusion is actually an illusion:
If a creature uses its action to examine the sound or image, the
creature can determine that it is an illusion with a successful
Intelligence (Investigation) check against your spell save DC. If a
creature discerns the illusion for what it is, the illusion becomes
faint to the creature.
What is unclear to me is the meaning of "faint". English not being my first language I checked the dictionary and what I understand is that my object will be blurry, out-of-focus, looking fake… but it will still be there. On the other hand, my GM has ruled it is simply transparent for whoever looks at it (context was putting a 5-ft-by-5-ft black box in front of a window to block the view).
Does seeing through an illusion make it transparent?
Best Answer
Yes
If something is barely perceptible, you will only just be able to see it. At a glance, you could miss it. This would not be true if the illusion remained opaque, so it makes sense that it would be transparent once you discern it for what it is.
From a gamist perspective, your question notes one of the main uses of Minor Illusion - to block vision. If the object remained opaque, in a lot of cases succeeding on that check would provide no benefit to an enemy.