This blue backpack is heavy.
I see that the English sentence has a parallel Spanish translation which uses ser. My thinking is that the choice depends on whether the rucksack itself is heavy (→es pesada) or whether the current load is heavy (→está pesada).
The first thing that came to mind when I read the sentence was that it was an error. The second thing was that maybe you’re weighing some bags and ‘this one has already been weighed’.
I’d say ‘es’ for both of the meanings you give, maybe ‘está muy cargada’ etc. if I mean precisely the latter. I see where you’re coming from but it ‘está’ sounds really bad to me for the translation.
Interesting! How about using estar to express your feeling when you’re lifting the rucksack: “Esta mochila, ¡qué pesada está!”?
That would still sound awkward to me. Instead I’d say ‘cuánto pesa’.
Then I suppose “Esta mochila azul pesa mucho” would be a fine, idiomatic translation.