The relationship between ao and aoi are not totally different like "近い - 地下/地価/治下". Aoi/Shiroi belong to the special group in i-adjectives that the stem itself can be used as a noun. They are quite few.
青い、赤い、白い、黒い、黄色い、茶色い、丸い、四角 い、、、
Anyway, hiragana/katakana are phonograms. aoi = あおい, aoi ≠ あお.