You can't do blending directly in PQ space, which means you have to create a scene render target in linear space and use shaders to convert PQ texture data to linear, etc. All of this is out of scope for the SDL 2D renderer at the moment.
This allows color operations to happen in linear space between sRGB input and sRGB output. This is currently supported on the direct3d11, direct3d12 and opengl renderers.
This is a good resource on blending in linear space vs sRGB space:
https://blog.johnnovak.net/2016/09/21/what-every-coder-should-know-about-gamma/
Also added testcolorspace to verify colorspace changes
Sam Lantinga