I got a little carried away here, so here's a quick summary.
TL;DR: Your yeast was probably either A) nutrient starved, B) Fermenting at too high of a temperature, or C) a combination of both. Regardless of your temperature control situation, I think A (nutrition) is the most likely cuplrit here.
While it's true that some yeast strains are more prone to SO2 production, this can be avoided altogether with proper nutrient additions and temperature control.
First, when your mead starts smelling sulfury, try adding more nutrients. There are several threads regarding this on GotMead and HomebrewTalk. This is also hinted at in the BJCP's list of mead faults:
Sulfury Hydrogen sulfide, sulfur dioxide. Rotten eggs, burning matches, and other sulfur-based aromas and flavors. Generally unpleasant.
Provide sufficient nitrogen-based nutrients. Check for infection. Check water for excessive sulfates. Check yeast health. Check for yeast autolysis (mead left on yeast too long at warm temperatures). Try another yeast strain. Cut back on sulfite additions.
If you are more interested in why I think nutrition is likely the culprit here, I've included a detailed breakdown below. If not at least remember this: If you followed the directions for the number of tsp per gallon on the Fermaid K + DAP, those are likely meant for wine, which already has a large amount of free amino nitrogen in the must itself. Since honey is pretty nutrient deficient, it will need way more.
Another consideration is your fermentation temperature. D47 is a pretty finicky yeast, and maxes out at 68 F (20 C) before it starts throwing crazy off flavors and smells. Remember, that's fermentation temperature, not ambient. Yeast metabolism throws off a lot of heat, so once you start hitting the 5 - 7 gallon range, internal temperature can become an issue.
Keep in mind though, the sulfur will probably age out, or you can splash rack it or try the copper trick. I've heard you can either rack through a copper scouring pad or throw either said pad or a length of sanitized copper tubing into your carboy for a week or two.
There are a couple of tools we can use to determine if you were under your recommended YAN for your batch.
There are some other calculators out there, but for this, I'm just going to keep it simple and use the first two.
First, using the GotMead mead calculator, we can estimate that your 20 lbs of honey + 12 lbs of blueberries would hit an SG of around 1.128. Assuming your D47 pushed all the way to its 14% tolerance, that would leave us around 1.021 as a final gravity.
Now, if we head over to the MeadMakr Batch Builder, you can adjust the sliders to match your approximate values from the mead calculator:
Batch Volume: 6 gallons
Yeast ABV: 14%
Nutrient Regime: Fermaid K/DAP
Target OG: 1.124
Starting Brix: 29
YAN Provided: 250
Honey Needed: 21.3lbs
Dry Yeast Minimum Weight: 12g
# Dry Yeast Packet(s): 3 (15g yeast)
Fermaid K: 11.4g
You can see in that calculation, to achieve 250 ppm YAN, you would need 11.4g Fermaid K and 21.6g DAP if you were using Go-Ferm and pitching at a higher rate (we'll ignore those parts right now to keep it simple). If we follow the approximations from this thread and this thread on GotMead, then that would be 2.85 tsp Fermaid K + 5.4 tsp DAP, for a total of 8.25. This is more a little more than double the amounts used in your particular batch. Sure, your blueberries definitely added some FAN, but there's no way it was enough to make up the difference.
Now, I was just using one of the built-in calculations, but if you wanted, you can head over the The MeadMakr Advanced Nutrient Calculator and input your ratio of Fermaid K:DAP. Here's an example input for a 2:1 Fermaid K:DAP ratio:
Total ppm YAN: 250
Must Volume (Gal): 6
Fermaid O Effectiveness: 0
Enforce Limits: Unchecked
Configure limits: Unchecked
Fermaid K: 66.67
Fermaid O: 0
Output total g to add
DAP: 9.01 (~2.25 tsp)
Fermaid K: 37.86 (~9.47 tsp)
Keep in mind, if you are following a staggered nutrient regimen, those are total amounts per batch, not per addition.