Mixing game systems (Blades with something else) is great, Great, GREAT! Let me give examples
I think it was @timdenee who posted about playing Ghost Lines with his group. That may be the obvious choice of a different (game-)system using the same background/lore with the chance to re-use NPCs, background, places, etc. There’s an even bigger advantage, which I will decribe a bit later, but first…
We did that lately a game of For the Queen for our Blades in the Dark group (and even got end of session xp out of it for our PCs). I posted a bit of background of our crew of assassins here if your want some details about oru crew.
We started into our BitD campaign with one dark secret we’re still playing to find out: What happend that we fell from grace from our privileged postion at the Leviathan Hunters, what happend to our leader, why did we survive where others had to die?
So, after some scores (and some parallel classic games of FtQ) our awesome GM Matthias proposed that we play FtQ in our BitD game as a big flash back into the past: It was more than great.
First we found out that our boss was not a guy but a lady and that was only the first insight this great game managed to bring to the table. It was unexpected, it was very cool, it was fiction-driven and it was collaborative story telling. It showed us so much - sometimes surprising - details about our version of Doskvol and the Imperial City, our PCs and NPCs, it was sheer mindblowing. Oh, and great fun too btw.
And this my friends is from my pov the major advantage of using a different gaming system inside your BitD universe: A fresh view on the story and the plot lines. When you think that your group’s storytelling is already good or great, give another system a try. It may boost your game and make you smile a lot.
Did you do something similiar in your games? Let me hear about that!