A BlogCast is simply a very short (typically five minute) webcast that's hosted on a Blog. The idea is that we're all busy people who want specific information and don't have the time/patience to sit through lengthy webcasts to find what we're looking for.
BlogCasts are rather like "Elevator pitches" in that they're concise, to the point and compel you to take action.
You may be familiar with the term PodCast, a BlogCast is different as it comprises video rather than simply audio. In addition the content can't be consumed by an RSS aggregator though the blog that hosts it can.
BlogCasts typically focus on actually demonstrating HOW to set up technical systems - a great example can be found on Eileen Brown's Blog.