So if ג'ינס refer to a single pair of jeans, and ג'ינסים refers to multiple pairs, then this sentence means that she is wearing multiple pairs of blue jeans? Sounds uncomfortable and/or like very creative styling. But really, does ג'ינסים sometimes mean a single article of clothing?
Someone else can provide a linguistic answer, but it's not uncommon to have more than just the ending change when going from singular to plural. E.g.:
Dog,s kelev, klavim.
Day,s yom, yamim.
Night,s laila, leilot.
Daughter,s bat, banot
It's not as if we didn't have that in English, too. Woman, women. Man, men. Goose, geese. Leaf, leaves. Mouse, mice. House, houses.
Why "ke" at the beginning. Here you have like a small apostrophe next to ג' which changes the pronunciation from "g" to "j". And ים is there because it's plural.
As for the pronunciation, I wouldn't write it "jeenseem", because you might want to pronounce the "ee" long, like in English "jeans". However, Hebrew has teo very short "i" sounds, like i in sit - jinsim.