My first job after graduating with my CS degree involved developing with an obscure language. Not unique. Not in-house developed. But an obscure enough VMS 4GL that I never saw anyone else using it.
In retrospect, this was an unwise decision. While I learned a lot about professional software development doing that job, it would have been a whole lot better to also develop a marketable language proficiency to go along with the "soft" skills.
I got lucky - my second job was working with another obscure VMS 4GL. They hired me because they knew they couldn't find people who already knew the language - the fact that I had experience on the VMS platform with a 4GL was enough. And at that job, I had the opportunity to get trained in a marketable language as they looked to move away from VMS towards Windows servers.
But I could easily have not had that luck, and found myself five years into my career with absolutely zero experience that would persuade a recruiter or HR person to look at my resume.
For your first job after graduation, I really, really don't recommend it.