While reading an article about history and use of the camera obscura and camera lucida the use of camera obscuras for the plural felt increasingly wrong. (whinge over)
In general when a (foreign) noun-adjective phrase is used in English the noun take the plural (aides-de-camp, adjutants general, etc.)¹. TFD cites "Random House Kernerman Webster's College Dictionary" for the plural camera obscuras, which is also given in wiktionary without citation; no other dictionaries online list a plural.
Of course, camera as an English word derived from the Latin camera = chamber has the plural cameras, but that doesn't mean cameras obscura would be right because in the phrase camera is still Latin. So should we be using camerae obscurae? This is stated in wikipedia without citation. Have I even got the plurals right in Latin? Probably not, I haven't studied Latin for nearly 25 years. Should we semi-anglicise it to cameras obscura?
¹There are many more words and phrases which take the plural in the middle in the answers to this question.
"Which style of Latin plurals should I use?" is relevant but doesn't provide an answer to what we should do in the case of a phrase. I would even say that it might support "cameras obscura" which nothing else does.