Here are some options, depending on the nuance you want to convey:
Literal translation:
* कृष्णफलक (krishna-phalaka): This literally translates to "black board".
* श्यामफलक (shyama-phalaka): This literally translates to "dark board".
Descriptive:
* लेखनफलक (lekhana-phalaka): This means "writing board".
* शिक्षणफलक (shikshana-phalaka): This means "teaching board".
* कालेयफलक (kaaleya-phalaka): This means "black-colored board".
Figurative:
* कालेयपट (kaaleya-pata): This refers to "black cloth", which could be used as a writing surface in the past.
The best choice depends on the context. For example, if you are talking about a blackboard in a classroom, "शिक्षणफलक" (shikshana-phalaka) would be a suitable option.
It's important to note that these words might not be commonly used in everyday conversation, as the concept of a blackboard is modern.