5e hiding rules are quite woolly, but their is no specific rule for spell effects to interfere. Your DM might rule that you need bigger cover to block LoS.
That I agree. There is no rule, so the DM has to make a ruling.Its not really a rule, its a ruling.
The DM might allow the player to fluff the spirits as shadows if it fit the theme of the character.There might be corner cases when you could, but personally I wouldn't allow it most cases.