This is probably because your layer is brought through QGIS by default with JPEG. JPEG does not have transparency support, so when there are areas with no data, they are displayed as black. We recommend that if you want to look at a zoomed out version of Nearmap imagery, for example when you're looking at a whole city or state, bring it in as PNG. If you're working mainly with zoomed in imagery, bring it in as JPEG. 

You can change the image format in QGIS to PNG by going to the Layer Properties Display, and selecting the image format you want.