The blem blocks I have seen for sale on ebay have been by the same seller and he points out the defect and seems to be up front about the situation ... and the price affects it.
Like it was mentioned .. most are porisity issues that wont affect performance, cosmetic blems.
Here is a link to one of his typical blem ads ...
http://cgi.ebay.com/ebaymotors/New-D...em110224721337