Although PowerPoint has ways to annotate the screen, I personally find it cumbersome to draw with the mouse. I could use a stylus with the bamboo pad and sometimes there are other input options.
But how to get that into a blog? Sometimes I use the program ExplainEverything for the iPad by itself. I can just upload that directly into YouTube.
If I want to intersperse PowerPoint presentations with diagrams and other kinds of modeling, I can use Reflector. This is not just a Mac program, but I have not had success with it using the PC, which is why I have it on the "Mac" page of the blog. Reflector accepts video feeds from"AirPlay"(this is an iOS method of sending video and sound to other units, like the AppleTV). You need to run the reflector program on the computer which is to receive the signals. I have it running on my laptop. I can use ScreenFlow to capture the desktop image and when I want to show something on my iPad, I can just activate the reflector program.
Another amazing feature is that you can consolidate several video feeds. For example, you can get up to (I think) eight student iPhones/iPad on your screen during class time. I occasionally use this to challenge the students to demonstrate a concept. I can keep some of the video feeds on a separate monitor and pull them up as interesting things develop.
If you typically use a Windows computer, you could try this program out. Like I said, I did not have luck with this.
You can purchase Reflector from AirSquirrels.