If spironolactone (for acne) is androgen blocker - how does this not have any interaction with androgenic progestin such as levonorgestrel/mirena?

It could: The circulating levels of Progesterone secreted by Mirena (levonorgestrel) are very low making androgen side effects possible but uncommon. Spironolactone would decrease the androgenic effects if they were to be present although a doctor would not typically treat Mirena (levonorgestrel) side effects this way. If I were the physician treating someone with androgenic side effects of Mirena (levonorgestrel) I would just change birth control methods

It depends: Spironolactone is indeed classified as androgen blocker. it's actual pharmacological effect depends however on environment (patient/condition) it is being used. In fact it often times it behaves as partial antagonist- agonist in presence of other potent steroids.