I have the game on steam but can't buy the DLC there. Is the itch DLC compatible with the steam version?
Viewing post in Countess in Crimson: The Black Dawn DLC comments
The game itself is compatible, but the installation process is different. I've only recently discovered that you can add an option to add Steam keys to itch.io purchases and I'm in the process of implementing this feature.
I'd recommend downloading the Steam key and activating on Steam to avoid the hassle. The keys should be available within a few days.
Update: Steam keys are now available.