The Blocked time includes any pre-processing time (such as cache lookup) and the time spent waiting for a network connection to become available.
Modern browsers will only create a maximum of six concurrent network connections per host name (such as www.microsoft.com) and will queue up requests until a network connection is available. The effect is even more pronounced on older browsers, such as IE 7, that only support a maximum of two active requests per host name.
Often the Blocked time is the most significant factor in the download time for images embedded in a web page.