Depends on what you mean by RPG.
Lots of people host CityRp's, which in turn are actually RPG's, as they use a RolePlayingGame mechanic.
However, these have a tendency to be poorly built, poorly moderated and can quite often spew idiocy.
Most RPG's tend to be Medieval RPG's, actually, and do work on the RPG basis of, click stuff to collect and level up.
These servers are generally run via use of Variable Events, so, the host will have put a lot of work into getting it to run right, so you can generally expect the server to be well built and moderated well, as no one who's put enough effort into eventing everything wants a terrible server.
The problem with searching for specific server types, is that most servers are not dedicated and will shut down when the host leaves.
Since most people who play Blockland are teenagers, they therefore have places to go, such as school and quite often turn off computers while at school, or away from home or sleeping, so, a lot of servers have varying times that they're on.
You can find good RPG's occasionally, but due to the effort put into them, they're harder to find and are much rarer than the all too common, Family Rp and CityRp.
The only real thing you can do, is just whenever you get a chance, search the server list for any RPG servers. There's little more to finding a good one than looking at the right time.
If you can't find any, then just have a look around other servers, or if you can, start your own.
So, put simply, there are RPG's in Blockland, but they can be hard to find and will have to be manually found.
So, look at the server list whenever you can and if you spot one, join.
Otherwise, just enjoy yourself however you can.