Do you have any specific requirements that would make it "block diagram software" instead of just any drawing application that can draw labeled boxes with arrows/lines between them? Seems like any Visio-like software would suit your needs. There are lots of free online options (
www.lucidchart.com or the Google Docs drawing module). If you're looking for an offline application, OpenOffice has a drawing module that is like Visio. If you're more Linux-oriented, XFig is a classic vector drawing application that has often been used for electronics drawing.