Hi Benzene and welcome to our Community.

Unfortunately I do not have the exact answer to your question, but my guess would be that there's no room in the timetable to allow for a stop here, as well as the fact that there's two other Intercity's that stop here. This train does not stop in Schiedam Centrum either, presumably for the same reason.

Hi Tomas, thanks for your reply. Ah, it is a pity. 

Is there any chance that the schedule will be changed in the new year and this stop will be added to the route? I ride this route every day and see quite a few people who, like me, have to travel from Blaak to Central to take this train. Maybe it would be possible to put this issue on some sort of travelers' poll? 

I will pass on your wish to the relevant department, but that is not a guarantee that this will change. Lets hope this changes in a positive way for you. 

In the weekend, there is one train to Eindhoven per hour that stops at Blaak. Apparently, this is not possible during weekdays.

That was only this weekend due to maintenance works between Rotterdam and The Hague. 

Rotterdam Blaak is an interesting case. According to NS it is an Intercity Station but not all IC trains stop there. You would think that because Blaak is an important public transport hub all IC trains could stop there. 
It could of course be that there simply are no suitlable train paths available to accomodate an extra stop as it could disrupt the timetabling on the HSL.

There's an obvious reason why the Eindhoven - The Hague intercity doesn't stop at Blaak: it departs Rotterdam Central for the Hague three minutes before an Amsterdam bound IC departs. 

As there are only two tracks between Rotterdam and Rijswijk, this means the IC from Eindhoven cannot stop at Blaak as the spacing with the next train would be too short.

An extensive rebuilt of Rotterdam Central would be needed to enable using all four tracks between Rotterdam and Schiedam for the Hague bound trains. Otherwise additional stops are not really easy to add.

Intercity trains should not be making nonsensical stops at secondary or tertiary stations in a city where they already have a stop nearby. If NS did that, they would also have to change the name of the service to Interregio or to Sprinter. IC trains are for fast connections and it's good that this train does not waste time stopping in Blaak or Schiedam or any other smaller places.

No it’s not, see https://assets.paris-p.cla.ns.nl/stations/vertrekstaten/rtd2.pdf Intercity trains to Dordrecht do stop at Blaak.

Rotterdam Alexander however is an intercity station, as is Gouda. But not Woerden, Amsterdam Bijmer Arena, Duivendrecht, Driebergen-Zeist (where only some intercity trains stop).

Or Veenendaal De Klomp (where only intercity tains stop, but not all of them).

Blaak is a bit like Amsterdam Amstel (which is listed as actual intercity station) but not all trains stop there. For Rotterdam Blaak - Breda (and further), via Rotterdam Centraal is the fastest option.

It’s time that NS went back to the days when there were three train categories: Intercity, sneltrein (semi-fast) and Sprinter/stoptrein.
 

Some Intercity trains don't make enough stops and some Sprinter trains makes far too many stops. This third option is more logical. Especially in places where there are already a lot of duplicate trains running 4-6 times per hour.