I'm enjoying the game a lot, however the max resolution option in-game seems to be 1920x1080 which makes the game look a bit 'blurry' on my 2560x1080 monitor.

I tried looking for a config file in the game directory but didn't find anything so far.
Does anyone know a solution or do I just have to deal with it?

that's why i sold mine, 21:9 support is becoming like the sli support, you need to wait for unofficial patches or if the developeer actually gives a shit, later games, specially indies and japanese games are becoming a pain to find proper ultrawide support.

I'm back with 16:9 and is just sit and play.

I have 3440x1440 display. The game is stretched horizontaly and everything looks bad and blurry, the texts are hard to read due to this. I understand that not every game does support ultrawide, but it could at least allow me to set 16:9 resolution while NOT stretching the image. But this game seems to do scaling by itself and not outputing real 16:9 resolution, which makes my display unable to show black borders at sides ... this either needs support for 21:9 or possibility to use black borders at sides.

To be fair that's the first game I ran into problems with.
I enjoy 21:9 even if it means taking some extra steps sometimes.

On that note, I seemed to have fixed it.

There already is a mod/fix. Can't post links but it's called "Bloodstained: Ritual of the Night Ultrawide & Multimonitor mod 1.03"

edit: Can someone post another comment about this so I can mark it as the solution?

Nah, it's plenty of games that support 21:9 Aspect Ratio.
Also for productivity is awesome *o*