Does anyone besides my husband insist on adding an -ed to sour cream? Etymonline dates "sour cream" to 1855, but has no mention of "soured", so I don't think this is analogous to "iced tea" or "ice cream". Is this a regional thing? He grew up in New England, but English is not his parents' first language, so his accent is more Uncle Walter than This Old House.
After a quick Google Ngram search, soured cream appears to be used very little. Personally, I have never heard it used. On the product itself, (in Canada), the label declares it to be sour cream. There are 39.8 million hits for sour cream on Google, and 0.6 million for soured cream. If we change the Google Ngram to British English, soured cream's popularity increases, so I assume that this is mainly a British expression. Changing it to American English shows almost no results for soured cream.
Looking at the entry for sour (the verb) in the NOAD, I find the following definition:
make or become sour: [with object]: water soured with tamarind | (as adjective soured): soured cream | [without object]: a bowl of milk was souring in the sun.
Soured cream is a valid alternative to sour cream.
Looking at the Corpus of Contemporary American English, I notice that the most used phrase is sour cream.
The data for soured cream is not visible because the CoCA reports just one or two sentences containing that phrase.
Surely the discussion here is not about what is correct, it is just labeling. Personally, I grew up in the UK and have always called it soured cream because that is what the label said when it was introduced to me.
Soured Cream simply means it has been soured (made sour), past tense. This in comparison with cream that has not been soured.
The other use simply says what it is - sour. This, in comparison with cream that is not sour.
One correctly defines the process that the cream has gone through. The other correctly describes the state that the cream is in.
If I leave cream out in my kitchen, it goes sour, it is sour cream - I will not eat it.
If I buy soured cream then it has gone through a process of souring with a known outcome - I will eat it.