Actually a red ‘transfer chest’ would be great. Something like a deep red but similar texture as the portal chest. And both of them glow in the dark, make them more distinguishable from a painted blue/red chest.
In multiplayer it could transfer between your blockheads and not another players blockheads if they tried to look inside. As in, if one of your block heads put an item in a Transfer chest another of you blockheads will be able to pick it up but if a blockhead of another player looks inside, they will not see the item. They can place an item so another of their blockhead will be able to pick it up but you, instead, will not see their items. Just your own.
Either that or it can just have both transfer but also act as a safe that others can’t open?