This is Normal in IT
> Am I wrong to be "blackmailing" them?
You are not "blackmailing" them. You have a new marketable skill and you are asking for the "market rate". This is capitalism.
Why should they be allowed to get you at your previous rate, now at a significant discount, when they can't get ANYONE to fix it at that rate? If they don't pay your rate, someone else will make an acceptable offer if you look for more opportunities. This is how we IT professionals survive and thrive.
If you want to know how I survived, and thrived in IT, since the mid 90's, let me know. It's a bit long for a forum posting.