The customizing guide section titled "Blocking issues that depend on other issues" needs updating.  Where it says this:

<span tal:replace="structure python:db.issue.classhelp('id,title')" />

It should say this:

<span tal:replace="structure python:db.issue.classhelp('id,title', property='blockers')" />

That same block of example code also needs a </td> closing tag.