Most of classic literature is in the public domain, as such it can be reprinted by anyone, or even altered from it’s original form (though that is a rather rude thing to do) without having to pay royalties.
This is one of the reasons comic book companies love such characters as Sherlock Holmes, Dracula, the Frankenstein Monster and Alice of Wonderland. (That and they’re great characters too!)
It’s the reason so many different companies have their version of these characters. If you do a comic with Captain Jack Sparrow, you have to pay the Disney Company a royalty and can’t change the character. You do Long John Silver, you have carte blanche and don’t have to pay Robert Lewis Stevenson a nickel.
There are also many comic book characters considered to be in the public domain and as such have been used by other companies and writers while also being changed vastly.
In this instance its probable these properties are really still in copyright as the 75 years needed for them to pass into public domain has not passed yet.
It’s just that due to the mercurial nature of the early comic book industry, with companies coming and falling off the radar yearly, the chances that someone will come forward with proof they own little known and/or unprofitable characters such as the Blazing Skull, The Black Terror, Rio Rita or the Rook is unlikely at best, and probably not worth the effort anyway.
Sometimes, a company may take a character in the public domain, change them and copyright the new character. Thor from Marvel is a good example of this.