I got bitten in the garden a week ago. the area swelled up and itched a lot,ithought it was getting better. but today, its itching and turned black?

Black bite: i am not sure where you live but in the southern ares of the US could be a brown recluse spider. In any event would follow up with doc/ ER to evaluate as black is usually a sign of necrotic ( dead skin) . After mosquito bites you can get hyperpigmentation but it is not black. https://www.emedicinehealth.com/insect_bites/article_em.htm#insect_bite_symptoms