I’m assuming it’s that probationary black belt. Some karate organizations do it, calling it shodan-ho. In those organizations, you’re given a black belt upon passing the test and you’re recognized as a black belt immediately in your dojo. But you don’t receive your official certificate and other things (wherever applicable) until a standard time period of consistent training.
I'm familiar with that process. In our MDK system, you progress from geup ranks to black belt candidate, to black belt. Because 1st Dan in this system is a teaching rank, that candidacy period is used for 'how to teach' training.
But "poom" means kiddie black belt. A black belt candidate holds the rank of Chodanbo. The literal translation is 'half a black belt' and we do use a half red/half black belt (red belt being the highest geup rank...) to signify this rank. It's the terminology I question, not the practice.
Improper use of Korean terms is not at all uncommon. It's not a huge issue, so long as everyone in your school knows what you mean, but it does become an issue when you're talking to people from other schools, or if you actually train with Korean speakers.
For example, our school has apparently always used "Sabumnim" for the chief instructor, and "Sabum" for all others. This isn't at all correct. "Sabum" means 'teacher' and "nim" is an honorific meant to show respect. So referring to someone as "Sabum" basically means 'teacher I don't respect.' Correct titles for assistant instructors would be things like "Busabum" or "Kyosha" with "nim" added to show respect. Likewise, one would never use "nim" when referring to oneself. Doing so would be incredibly arrogant.