I wouldn't call it blackmail, but its a very precarious bargaining position.
Lets break it down like this, you are skilled in the very rare FooBar framework. and finding people that know this framework it hard.
To get a new developer who is skilled in FooBar might cost quite a lot of money, so you want an increase to $X0,000 dollars per year or you walk (where X is far higher that web developers in your area earn). That seems like a smart proposal, but it can backfire on you.
The company can look at how much developers generally cost in your area - lets call this $Y0,000 per year. Now if X is far higher than Y, they may consider that they'll get a new developer at Y and just take a hit on their performance while they get up to speed.
Also, you need understand you aren't really worth $X0,000 per year. If you walk out the door today, you can't get the $X0,000 that you are bargaining for because the FooBar framework is actually also very rare. So you may not be able to find an employer who needs a FooBar developer and will pay $X0,000.
Your knowledge may have put you in the position where you occupy a niche too well.
Information Technology has a poor reputation sometimes for having a wild-west cavalier attitude towards business. But there have been attempts to build codes of ethics around IT employment to help mature the branch of knowledge.
Ethically, I believe that it is irresponsible to be sole lifeline for a specific system for an employer. That doesn't mean that there should be two staff with equal knowledge at all times, or that the secretary could take over. What it does mean however, is part of your job is documenting the system to the point that if you were unavailable to work for an extended period your employer wouldn't go bust.
Nor does it mean documenting yourself out of a job. Knowledge and skills are transferable, but while it might be possible for someone to reproduce your work piecemeal from instructions, it may take them much longer due to your greater experience - something which isn't as easily transferred and is much more valuable.
What should you do?
Obviously you have a good bargaining position here, and there is no reason not to utilise it, but its unwise to try and take advantage of your employer. Look at the market in your area, and see how your pay compares. Also take into account the relative rarity of the technology and negotiate a fair raise on that.
Keep in mind that if you are the only one with the knowledge and have never documented anything, then if X is too high there is the risk that they may try and find someone else for the same price that will document.
Personally, I'd also recommend phrasing your position as "I have lots of experience in FooBar which is hard to find" rather than "I'm the only one who knows FooBar". The former demonstrates your worth, and the latter comes across as a threat, and may make them aware that they do have a knowledge transfer problem and they may want to solve that.