Ultimately, either way it will create a new competition which will push the hand of the legal market to adapt to being undercut. In that sense it's not simply just the individual benefiting at expense of the wider society but potentially the catalyst which benefits the whole market (even non-black market buyers).
There is no reason why Epipens cost $300 other than the fact the FDA has a huge backlog [1] and other regulatory constraints. There are plenty of companies chomping at the bit to deliver that medication for cheaper.
Like you said, this alternative is less than ideal, these hackers weren't driven to this just for fun - it was born out of what they saw as a lack of other options, a necessity (same with their 'customers' who were pushed to the shadier/riskier option). This necessity is generated when the primary market is not delivering value properly/efficiently, creating a demand for it.
The more free the legal market, the more the black market will be far less lucrative. The same applies to drugs and many other markets which artificial controls/limitations.
This all must be factored into the balances of costs measured against the externalities it imposes on society. It's riskier than pure top-down/government intervention but sometimes it's a necessary risk when that option continually fails to reform/change and nothing else pushes the hand of the gov/industry to adapt.
Sure you could blame the black market, but the simpler solution would be to fix the original problem which caused the black market to exist.
[1] http://slatestarcodex.com/2016/08/29/reverse-voxsplaining-dr...