We recovered a rather battered full-face motorcycle helmet from our church charity shop recently. As a protective motorcycle helmet it is now worse than useless, but for TUGs..........
We painted the helmet gloss black all over, including both sides of the visor.
The helmet can be safely worn as a hood, and a blindfold when the visor is down.
It doesn't gag the captive but it does muffle their voice.
Not when you've been wearing it for a while! LOL. It's a small helmet with a thickly-insulated interior, so its very snug fitting, and with the visor down it soon gets very warm! LOL.
Anyone who's ever worn a motorcycle helmet (even the more "open" design will know they get warm if they've been on for a while. We usually remove the "all-in-one" after about 1 hour and give the captive a drink - even if we don't untie them - just to keep their temperature down.