First you're going to want to get yourself a time machine, for this guide I will be using a DMC DeLorean.

1. Travel back in time to when A.V.A. still had live servers, below is a demonstration of me doing just that.
(Pro tip: try go back to a time when you weren't hope so as not to cause any accidental time paradoxes)

Now that you're in the past I suggest you go to another guide to learn how to actually get the knife, a good one is:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=159390497

If you are reading this you have now gotten the black rose in tf2 and hopefully caused no major rifts in the space time continuum, congratulations!