Complex equipment requires constant maintenance, and human know-how needs preservation. Within a couple of years of neglect, machinery can break down to a degree that only a major overhaul requiring expert skills can fix. Knowledge and skills transfer is a major, major issue till today in all kinds of corporate, technical and defence industries* - we still essentially follow a master-apprentice skill transfer paradigm, and skills degrade with lack of use. That's 2 potent weapons Blake would have had in his hand against any planets not conforming to his communications protocol. So did he wield it?
As a matter of course, the Star League Ministry of Communications had controlled HPG technology extremely closely, not allowing the Great Houses any technology transfers or even enough info to reverse-engineer their own HPGs. Just like Royal equipment, the League was already monopolising HPGs and HPG tech from the start. We can infer therefore that a lot of safeguards were already in place, including Blake picking ultra-loyal chief tech officers to run the HPGs, so that the Great Houses would not even be able to subvert a Comstar officer with the know-how to help them put together a HPG.
Nonetheless, maybe they did compromise a few officers and steal a few pieces of tech. Maybe Blake's counter-espionage success rate was only 80%, just like the SLDF - but 20% of a working HPG plan doesn't equal a working HPG. And in the nuclear fires of the Succession Wars that followed, probably the Great Houses lost what little they had managed to steal, just like all the other Lostech.
Don't forget that when ROM was founded they then probably did, er, retro-actively seal off any such leaks with kill squads and sabotage.
*For example, the Russian Navy has basically lost the skill to design large military vessels, which is why they needed to buy Mistrals LPDs from France. Meanwhile the Royal Navy is trying desperately not to go the same way, and pays through the nose to ensure the UK continues to preserve an indigenous warship design capability.