This is an interesting question that comes up often. While titanium is resistant to rust, it is not rust-proof.
A blade made of titanium will not necessarily rust, but in a place where the humidity is high, there is a risk that the blade can rust.
The idea of rust-proofing a knife blade is a bit misleading. A knife blade can rust, but it won’t rust through, and it won’t corrode as steel would.
Titanium is resistant to rust because the metal is non-ferrous, so it does not interact with steel components.
For example, a titanium frame lock knife will not rust because the blade is made of titanium, and the lock, handle, and other components are made of steel.
In a knife where only the blade is made of titanium, the blade will rust, but it will not corrode the steel components.
A rust-proofing finish on titanium knives means that the titanium blade has a layer of chromium, tungsten, or vanadium oxide on the blade.
This oxide layer protects the titanium from oxidizing, but it does not protect it from scratches or abrasions.
A titanium blade with a rust-proofing finish is not rust-proof; it simply has a layer of corrosion-resistant material on the titanium, which is non-corrosive.