I have never tried a Black Magic electric fan, but every electric fan I have tried, failed miserably at keeping the engine temperature down. Also, an OE style electric fan takes a lot of current to operate, if that dyno test was on an engine dyno, the HP used by the alternator load would not be accounted for. A good working fan may eat up some power, but not as much as an overheating, seized up engine does! Also, a heavy fan like a clutch fan, takes power to accelerate that mass, may not show up on a dyno pull, but certainly can on an ET slip. Still, 30 HP seems like an awfully high number lost with a fan, what RPMs were they turning? I would think a single fan belt would have trouble maintaining a good grip dealing with a 30 HP fan, not to mention the water pump and alternator drag on top of that.
Most of the electric fans out there, including those Black Magic fans, are not up to the cooling job, based on my own experience. However, the factory electric fans, and also Spal electric fans, cool better than any standard fan or clutch fan of equivalent size. Usually though, because the electric fans are smaller in diameter, you need two of them to cool properly, rather than just one big fixed blade fan.
My dyno testing does show that a fixed blade fan will eat about 10 HP at 5500 RPM on a typical street engine.
As far as power consumed by the alternator to run an electric fan, it is negligible. The calculation is straightforward, because watts and horsepower are a measure of the same thing, just using different measurement units. 1 HP = 750 watts, or 1000 HP = 750 Kilowatts. If you go to Europe and look at the new car specifications, their power output is measured in kilowatts, not horsepower. For electric fans, you need to know that watts = voltage X current. So, if you have two electric fans, each drawing 10 amps at 14V, you have 20X14, or 280 watts. That's about 4/10s of a horsepower. If the alternator is only 50% efficient, that means that that alternator is consuming 8/10s of a horsepower to generate the current to run those fans. That is much, much less than a fixed blade fan will take.
One reason I really like electric fans and electric water pumps is that they run continuously at maximum output, regardless of engine speed. In the case of a fixed blade fan, it has to be designed to provide enough airflow to cool the engine while the car is stuck in traffic, not moving, on a hot day. So, it has to be pretty big, and then when the engine is turning at a higher speed it takes a lot of power to turn that fan. The same thing applies with a water pump; it has to be sized to provide enough flow to cool the engine at idle, and that capacity is wasted at higher engine speeds. Electric fans and water pumps run at full speed, independently of engine RPM, so they do not contribute to parasitic horsepower losses at higher engine speeds. Electric water pump plus electric fans will typically save 20 HP at peak RPM on a 400 HP engine. Electrics are the only way to go, if you ask me...