Where does “блатьница” come from?

блатьница (Church Slavonic) comes from Church Slavonic блато, from Proto-Slavic bolto, from Proto-Balto-Slavic balta.

Ancestry of “блатьница”, step by step

StepLanguageWordMeaning
1Church Slavonicблато—
2Proto-Slavicboltoswamp; mud
3Proto-Balto-Slavicbalta—
Every word from Proto-Balto-Slavic balta →Every word from Proto-Slavic bolto →Every word from Church Slavonic блато →