I also prefer just single blank mainly for same reasons @ShixinZeng gave.
However for an alternative solution then how about #_
Because this then follows on nicely to having literal TRUE / FALSE...
blank: #_
true: #true
false: #false
And if not enough savings are perceived over #[true] & #[false] then these can be shortened (or have aliases) to #T & #F
PS. I know this is stepping further onto ISSUE! datatype but we already have Prebol, Include & Red macros doing this!