Madame Tussauds wax museum in London has the blade on display

Edit: Looks like it's only Marie Antoinette. Not sure about the others http://www.cultofweird.com/crime/marie-antoinette-guillotine-blade/

That is a wonderful question. Funny because I was at the Place de la Concorde earlier today having lunch thinking about death. I actually don't know, however I'm currently working on an article about the Guillotine throughout history, and in the course of my archive proweling here in Paris, I will be sure to sniff around and see what I can find.

However my best guess would be that it is lost to history. There was not any specific gilded or otherwise "unique" characteristics to the Guillotines used during the terror. They were built, moved, and assembled as a quick means to make a public example. I believe the actualy guillotine at the Place de la Concorde was not a permanent thing (the platform, however, was), and the same guillotine likely did not kill Robespierre, Marie Antoinette, and Louis XVI. This mobility suggests that it is lost to history. However as another poster suggested, perhaps the blade still exists somewhere in a private collection. In terms of how importance might be assigned to the execution, the physical wooden guide, rope, and pillion is unremarkable. The blade itself is what did the killing. It would make sense for such a blade to be preserved/saved, if at all.

However if I come up with anything in the next few days (I'm studying in Paris right now) I will report back!

After a rudimentary Google search, I found a picture of a guillotine blade with a plaque saying that it beheaded Marie Antoinette, and that it was purchased from the executioners family. However, it makes no mention of the King or of Robespierre, or of the blades history between then and now. Does anyone have a more in depth answer?

After having stood in the Place de la Révolution during the deaths of Louis and Antoinette, the position of the guillotine was moved around. I don't know if it was the same one that was transported back to that square for Robespierre's execution, but it's possible.

After Robespierre's death, it was moved around a bit more, and eventually ended up at Place de Grève, where some famous-but-not-THAT-famous people like Jean-Baptiste Carrier and Fouquier-Tinville were executed.

The reason I bring that up is that there are a few blades connected to the executions on Place de Grève. One is in the museum of the Paris Prefecture of Police (link with tiny picture), and the sign says that it was used in that place during the revolution. However, the guillotine was also placed there originally, in 1792, before the Revolutionary Tribunal decided to have it moved in 1793 to Place de la Révolution.

There's also the blade used to execute Carrier, which would have been in November, following Robespierre's death in July. It's been preserved, and here's a picture with some information. However, the fact that no one seems to mention Robespierre in connection with this blade makes me doubt that it's the same one. It's my best bet, though.

There's a blade on display in London claimed to be the one that killed Marie Antoinette, and purchased by the executioner's family. However, the fact that it was acquired for the Madame Tussaud collection makes me doubt it. The lady herself was a lying liar who lied, especially to create connections between herself and famous individuals. Considering that the exhibition is based on her work, I wouldn't expect her heirs to be much more attached to historical fact than she was.