I’m not a darktable user, but this sounds like something to do with the selected rendering intent. Reading “3.2.6 Color Management” in the darktable docs might give some insight. Essentially, you want to be using relative_colorimetric intent, as this will scale out-of-gamut colors toward thei white point, which preserves the hue. This might be all you need to do to get an acceptable rendition.
Now, if you don’t mind a bit of a story, extreme blue colors are challenging to most default camera profiles, which are usually 3x3 matrix profiles. All these profiles can do is describe out-of-gamut translations as a linear function, along a straight line, and to get the extreme colors to not posterize it takes a bit of curviness in the translation. A LUT camera profile does this nicely.
One expedient way to get a LUT profile for a specific camera is to get the Adobe Standard DCP for that camera from the Adobe DNG Converter. In RawTherapee you can just use it directly; for darktable you’d need to convert it to an ICC profile. I’ve done this successfully for my Nikon D7000, but the same thing for my Z6 didn’t work so well and I haven’t taken the time to figure out why, so YMMV…
I just looked for the 550D in my download of the DNG converter, and it’s there. To convert it to a LUT ICC profile, you’d need to get a copy of the dcamprof program (a really useful profile tool). I don’t know what your computer chops are (welcome to the forum, BTW…), but it’s a C program you’d have to compile, then run it in a command shell to do the work.
You’ve come to a good place to learn about all this; a lot of knowledgeable and helpful people post here. I’m not one of them, BTW, I just have learned all this in the past couple of years… 