From your description the bruise
seems to have come from friction between your boots and your foot. This can often happen without you noticing as you might have been concentrated in something else while putting on the boots. You need not worry, that will heal soon enough. Just make sure you wear some good cotton socks before you put on the boots. It will be better if you could put on an open air shoe for a while before you start wearing cover shoe, to give yourself some time for healing and prevent aggravation.