Reaaly quite simple
There was no "blast." A CME is a vast, outward ejection of plasma and magnetic field, but at a density lower than the best lab vacuum on earth. For fast CMEs, and the 2012 July 22/23 event was a very fast one, the front is moving faster than the Alfvén speed in the ambient solar wind plasma, so a shock front builds up. The front contained higher densities (by a factor of two or three) and concentrated magnetic fields, as well as charged particles (protons and some heavier ions) accelerated to high energies.
Nothing about those conditions is likely to affect a spacecraft designed and qualified to work outside the earth's magnetosphere. As far as I know, STEREO-Ahead suffered only a tiny decrease in solar array outage coincident with the shock passage, most likely due to the energetic particle damage to the silicon.
CMEs get dangerous to life in space when they feature shock fronts that entrain high solar energetic particle concentrations, and CME interactions with planetary magnetospheres can induce currents in (in the earth's case) the oceans and solid earth, preferentially in certain geologies for the latter, hence the danger to electric power generation hardware in certain places (e.g. Québec, which sits atop the Laurentian Shield) at high geomagnetic latitudes. The potential hazard from an historically strong event is that those effects could be seen at lower latitudes, but that is not known from historical experience. The record from the 1840s is that geomagnetically induced currents were experienced in telegraph lines as far south as the low 40s of latitude, which is also about the southern limit of telephone trunk line damage in the 20th century during events with under half the current-inducing potential of the 2012 event. I think it's fair to say we don't know what would happen with a ring current as strong as the one Prof. Baker and co-authors predicted for a 2012-like event hitting the earth's magnetosphere. (And from what it's worth, it's a 50-50 proposition: if the magnetic field in the CME is primarily parallel to the earth's dipole field orientation, it's a dud; only if it's antiparallel, as in the case of the 2012 event, do things get exciting.)
Another reason STEREO felt little, if any effects, is that the analogy to nuclear EMPs is a very poor one. The shock fronts from nuclear detonations are much more concentrated (thinner and denser) than one driven by a fast CME in the solar wind, and the magnetic fields invalided are much weaker. And even EMP effects on electrical systems can be largely countered by existing surge suppression techniques if one incorporates faster responding suppressors than most household surge suppressors offer.
Biting the hand that feeds IT
