Non-native learner here, the way I understand it at the moment would be that the partitive case ending is here used because mämmi is an uncountable noun. Extrapolating, perhaps if it was just "mämmi on musta" would refer to a single package full of mämmi that had been previously discussed?
I could be wrong about that, though
Essentially, but when olla is used as an existential ("there is"/"there are") such as in the have constructions, what follows it, which you may think is the subject, can be partitive too because it's not actually the subject but the predicate.
E.g. Minulla on mämmiä. I have some mämmi. Essentially this sentence doesn't have a subject. There is a locative phrase (minulla), then a verb (on), then a predicate noun, which is in the partitive (mämmiä).
I got warned about the umlaut (or whatever it's called - the two dots over the 'ä') when I typed the word "mammi" without it in English. English doesn't use umlauts, so it is never wrong to omit it in English. They can be included for accuracy, if desired, but they are never mandatory. At least that's my understanding as a native English speaker.
In German, Finnish, or any other language that uses the symbol, yes, I'd agree. My point is that it makes no difference whatsoever in English since the English language doesn't use them. The symbol has no meaning in English, and will be completely ignored (or worse *) by most speakers. You might as well dot your i's with circles or put a slash through your E's or a little star on top of your T's for all the difference it makes. :)
Only people like us, who know a little about languages that use such symbols, will make any attempt to respect them, anyway.
* - e.g. Wayne Newton warbling "Danke Shane" (instead of "Schön")
I might mention for the sake of completeness that in old books, you may occasionally see two dots above an English vowel, like in "naïve" or "coördinate", but that's called a diaeresis, and it has a very different function - it indicates that the two vowels are to be pronounced separately, but does not change either of their individual sounds. So far as I know, it is only ever found on the second of two consecutive vowels, and it is quite rare in modern usage.