it worked, so that's good.  And I think you did an excellent job of correctly reading weakness and making a difficult bluff after correctly assessing their hand strength. If you had a particular read to make this super exploitable bluff even better. 

BUT obviously as a range play this is pretty tough to argue for.  You have no blockers to anything (flushes, straights, sets) so you're basically just betting with napkins hoping they're weak.  You have good reason to think that's correct (and it was :-)!!!!) but you have to be careful with this kind of aggression.  Due this too often and you start getting looked up by sets and straights, OR they start slowplaying and rebluffing and you find yourself just spewing chips.  main things to considered when deciding whether or not to bluff the turn without any equity or blockers:

1) are you going to be able to bluff the river if the board changes?  if not then I'd wait for some blockers that will decrease the likelihood that villains have a slow play or can justify a bluff catch.  It's hard to make a profitable two street bluff when you have no equity, no blockers, and you need a blank to hit on the river.

2) can you credibly rep the nuts after the flop checked around/is this a locked down board that could result in a slow-play?  In this case the fact that the NF and the straight flush could both be present on the flop (with an SPR that only necessitates two streets of betting) worries me a little bit.  Either player could be slow playing on this board.  The good news is that so could you.  So this matters a little less.

3) does the number of players in the pot make you more or less likely to get looked up?  The less locked down the board is the tougher it is to run a bluff like this.  IE if there are no flushes and/or some non-nut straights or a board pair you're much likely to get bluff caught by trips or a weak straight in a multiway pot.  But on a locked down board when everyone has shown weakness up to the turn in a low SPR spot where typically you want to start betting and getting value/protection I think you have a better chance of repping strength than in a single raised pot where people are more likely to believe you are trying to bluff them out of the pot.  IE one of the best things you have going for your bluff here is that both players believe that the presence of the other player will protect them in this pot and lower your frequencies to bluff.  You actually look more credible betting into two people than one on the turn.  So when they both passively play the turn they're telling you that they're bluff catching or drawing. 

I think it's a good read and play, I just wish you had a least a Q or T of spades as a blocker to help monitor your frequencies in this spot.

cliffs: nh, but be careful doing this too much.

I like the bluff looking at it from a credibility perspective. Unless theyre stations, i think youll only get called by the Qflush or Tflush. The Qflush is prob never folding.

The only way i know how to calc the frequency of a certain hand(s) villain could be holding is Odds Oracle or Pokerjuice.

Im on my phone atm, if youre really interested in the frequency they have a flush, or the two biggest flushes, post in this thread or pm me and i can run a quick simulation given you provide me (a guesstimation of) their stats...

i like it! i think you have to get huge credibility for a flush here