iOS app icon with transparent background showing black background on device

On IOS use just square icons, if you use rounder icons you will have black margin black background.

From the apple developer website after a quick search:

Avoid transparency. An app icon should be opaque. If the icon’s boundaries are smaller than the recommended sizes—or you use transparency to create “see-through” areas—the resulting icon can appear to float on a black background, which tends to look especially unattractive on the beautiful wallpapers that users choose.

https://developer.apple.com/ios/human-interface-guidelines/icons-and-images/app-icon/

Edit: Also, check the related questions before you confirm your post - Is is possible to use transparency in an iPhone app icon?