Well Colby, two females shouldn't ever Hook up...granted it does happen in Humans so one never really knows for sure.
My main thought is that with so much more information becoming available, we're starting to find that more and more of the marine fish are Hermaphroditic, either sequentially, simultaneously (Hamlets), AND/OR they actually do have distinct sexual dimorphism. Yes, in Triggers and Lionfish..at least with what I know about them a blood test for sexual identity probably WOULD be helpful.
Yellow Tangs? Again, from my understanding of what yellow tangs need as far as space for breeding is concerned, the amount the setup would cost I don't think the Yellow Tang breeder is going to have any qualms about purchasing a dozen or more tangs to breed with...that thought is probably applicable to just about ANY tang species, at least from a monetary standpoint. I'd suspect the same type of circumstance might arise for the average hobbyist attempting to breed lionfish. Triggerfish, Puffers, maybe there's the useful species.
The problem is that many of the fish that could benefit the most from this type of sexing are SMALL if not TINY. I'm thinking gobies that aren't hermaphroditic, firefish, cardinalfish, blennies (who actually HAVE dimorphic charactaristics), jawfish (again, in theory these do exhibit some dimorphism even when not sexually active), certain wrasse species that do NOT exhibit dimorphism (i.e. 6 Lines). MOST of these fish can be purchased for less than what it would cost to have their blood tested (assuming the costs would be in line with the $20 I found for sexing a parrot), let alone the fact that the average hobbyist is going to have a pretty hard time extracting blood from these fish. Unfortunately I think DNA/Chromosome/Blood testing isn't nearly as viable as it is in the avian arena...most likely any blood test for mariculture would be an industry type thing.
BTW, not showing off, but just trying to illustrate a point. Here's my current success rate on pairing up fish using only existing methods:
Synchiropus splendidus - successfully paired using dimorphic characteristic of elongated first dorsal spine in males.
Synchiropus stellatus - "ditto" although in this case the differences between male and female dorsal fins are much more dramatic than in the Mandarin.
Pterapogon kaudernii - successfully paired by purchasing 5 juveniles observing behavior and removing extra juveniles once a pair made their precense known.
Apogon leptacanthus - successfully paired by purchasing 3 individuals displaying different dorsal fin lengths and overall sizes..only later found out that in our group the FEMALES have the elongated dorsal spines and are the dominent fish in this small group
Hippocampus barbouri - successfully paired by purchasing a pair...male has pouch so it's a no-brainer anyways
Amphiprion percula - successfully paired by purchasing 2 juveniles of disparate sizes...sequential hermaphroditism takes care of the rest.
Gobiosoma multifasciatus - didn't set out to pair them up, but purchased 2 juvenile Captive Bred Greenbanded Gobies. Haven't seen conclusive evidence or even heard a definite statement of fact, but it is assumed that these are sequential hermaphrodites.
Firefish - most likely successfully paired by observing individual interactions amoung a group at the LFS. They get along, share a bolt hole, and overall act like a male/female pair of firefish is supposed to act....just waiting for larvae to prove that they are actually a pair.
Apogon margaritophorus - got LUCKY, purchasing all 3 in a LFS tank when a male was discovered holding a batch of eggs.
Ecsenius graveri - still not 100% sure on this one...simply ordered 2 from an online source, asked them to do their best to try to get disparate fish, and well, we had 2 spawns that didn't last long enough to determine if we have a male and female. Based on behavior, it would appear that we do have a pair...so it was a 50% gamble on purchasing only 2 fish.
- So to sum up my experiences, yes sometimes there's a bit of "luck" but for the most part MOST of the fish hobbyists are currently working with can be "sexed" or more appropriately paired by either having an understanding of the reproductive biology OR through observing the interactions between fish in a larger group. I just don't see the ability of testing all these fish as being a viable option for the HOME aquarist.
Where the real opportunity for something like this to be developed and put to good use would be on the business side of things, mostly in mariculture but also for a vendor who wanted to reliably pair up as many fish as possible in order to cater to breeders.
Just my $0.02.
Matt
Think you should have to sex the blues dotted to then see his breeding dress 
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So going full circle, it would be difficult to show that need/ use that gets the whole thing rolling 
JMO based on extensive discussions with a biotech entrpeneur.
