I just finished a game using the French Advance variation and played this move which, from searching using the opening explorer and online, doesn't seem to have been played by anyone else. Is this because it's unsound [and if so in what way] or are there simply better moves to play in the position?

its a strong novelty.

What if Black plays 9...Qc7 instead of 9...Bc5?  Then White has to decide if he wants to lose his e-pawn or his c-pawn (both with check).

woah i know this is offf topic but why have you stop making those very useful blogs i thought you didnt even play here anymore please make more.

Glad you liked my articles (not blogs).  They take a lot of time to write well (unlike a blog post) and I have had other things going on lately.  I was just thinking about writing one about the games of a tournament I won a few weeks ago.  Maybe I will now, due to overwhelming demand!  

ohh trust me im far from the only one there been people ive talked to about it in the past. Great to here you coming back.

Posted this 3 years ago, wake up today to find people have commented on it.

spassky, the c-pawn is defended by the b1 knight, so only the e-pawn needs to be defended. 

General impression of the position after 9.Nxd4:

e5 and c3 are weaker.  The development of the b1 knight is somewhat impeded.  Black's natural french queenside play seems accelerated (the backward pawn is a target and black can immediately break with a6 if he wants.  And by the way, the knight on a5 seems as useful as the knight on d4.

Pfren sums it up by saying all it does is create pawn liabilities.

+1

Instead of advancing his development with ...Ne7, Black chooses to lose two tempi, giving White at least some reasonable compensation for his pawn sac in the process - it makes no sense at all to play ...Bxd4, an unforced error.

I used to play french defense. But i usually play 6...Nh6 instead of 6...Bd7 but i will check this variant...it seems to be interesting...

 I know that feeling.

What's depressing is that I stopped playing chess for those 3 years and I'm now worse than when I made this thread.