The Egyptian Gods and the Nile

All of those various elements of creation in one way or another were iconic of the Egyptian gods at the time. So when he when God transforms that into into blood, this is his way of basically saying, well, I'll show you what a puny God that Egyptian God is because I can turn his waters into into blood. He does that by using that which he has created and which idolaters have deified to show that it is nothing more than something of his own creation and thus under his under his power.