I assume that you mean 'murky', which I've always thought of as being part of standard English. (e.g. "He has a somewhat murky past" or "It's looking very murky outside today").
'Murk', as a noun meaning darkness or gloom, was first recorded in English usage in 1313, in the form of the Old English 'myrce'. It's now largely confined to use in Scotland or in poetry. When meaning thick mist or fog, it was first recorded in 1400.
The adjective, 'murky', is in more general use, with the following definitions offered by the OED:
1. Of a place: dark; gloomy. (First recorded in 1590 but citing a 1340 reference).
2a. Of air, the atmosphere, etc.: obscured by mist or vapour; foggy, cloudy. Of mist, clouds, etc., or darkness itself: dense, thick, intense, impenetrable. (First recorded in 1667).
2b. Of water, etc.: cloudy due to suspended matter; dirty. (1840).
2c. Of a photograph, film, or other visual image: indistinct, unclear, blurred. Of recorded sound: poorly reproduced. (1874).
3. Dirty, grimy. (1755).
4. Dark in colour. (1759).
5. Shady, suspect; morally questionable, sinister. (1779).
6. Obscure, confused, imprecisely defined. (1783).
7. Of a look, a person's demeanour, etc.: sullen, cheerless, gloomy. (1830).