TFS2013 Requiring Comments on transitions to certain states - tfs

Question

I'm creating a TFS 2013 Kanban board. I want to require an additional comment when the user switches the State to "Blocked From Progress"
Based on this post (http://social.msdn.microsoft.com/Forums/vstudio/en-US/9b672f4c-3054-481a-856d-4da650f25f74/how-to-require-comments-during-state-transition?forum=tfsprocess) I can require comments on all state changes. How do I limit it to just a transition to the "Blocked From Progress" State? Here's a section of code from my exported User Story xml file:
<FIELD name="HistoryHelper" refname="Demo.HistoryHelper" type="String">
<WHENNOT field="System.State" value="Blocked From Progress">
<COPY from="value" value="1" />
</WHENNOT>
<WHEN field="System.History" value="">
<COPY from="value" value="0" />
</WHEN>
<WHENNOT field="System.History" value="">
<COPY from="value" value="1" />
</WHENNOT>
<WHENCHANGED field="System.State">
<PROHIBITEDVALUES>
<LISTITEM value="0" />
</PROHIBITEDVALUES>
</WHENCHANGED>
</FIELD>
This is almost exactly the code from the above link, but I've added a check for the value of the State. Those lines don't seem to change the behavior at all. I've tried them at different places in the XML to no avail. Also, I've tried nesting a inside a and got errors. I don't know if there's a way to do an AND inside a WHEN.
EDIT
In addition to the accepted answer, I updated the new Field definition to be as follows:
<FIELD name="Blocked Reason" refname="CustomerProjects.BlockedReason" type="HTML" >
<WHENNOT field="System.State" value="Blocked From Progress">
<COPY from="value" value="" />
<READONLY />
</WHENNOT>
</FIELD>

Answer 1

I think a good solution for this would be to create a new field called "Blocked Reason" and then make it required on transitions into the "Blocked From Progress" state. You'd implement this like:
<Fields>
...
<FIELD name="Blocked Reason" refname="<company>.BlockedReason" type="HTML" />
...
</FIELDS>
<WORKFLOW>
...
<TRANSITIONS>
...
<TRANSITION from="<somestate>" to="Blocked From Progress">
...
<FIELDS>
<FIELD refname="<company>.BlockedReason">
<REQUIRED />
</FIELD>
</FIELDS>
</TRANSITION>
...
</TRANSITIONS>
...
</WORKFLOW>
You'd need to set that for each possible transition into the "Blocked From Progress" state. If you wanted you could also remove the text from the field using the tag on transitions out of the "Blocked From Progress" state.