Problem
Gradients have bothered me forever, not only Affinity but every piece of software that has them. Turns out that both gradients and blur are subject to the same problem, which is that they are not performing the color average calculations correctly.
The issue is best explained in this video.
The gradient is on the left and the blur is on the right. The color between these two is too dark. This happens for every color combination. You can also see the edges of the gradient because it is being calculated linearly.
The fix below using gradients is still wrong because it still is victim to the darkness resulting from the bad average calculation. This is also a good example of the linear gradient issue because you can see a definitive purple line down the center.
This is my best attempt to fix the linear gradient and make it more logarithmic
This calculation issue is also extremely visible in the color chooser
The examples above are all using Adobe RGB, not sRGB.
Suggested fix
Idea 1
Because many documents are now using the wrong calculation, my suggested fix would be to add an option to the gradient tool for what type of calculation to use in order to maintain compatibility with older documents.
Idea 2
Take the Photoshop approach and just make it a document setting (and turn it on by default for new documents).
Result
Fixing this will result in a more accurate and pleasing gradient and blur tool.
Fixing this will also fix the linear gradient problem, which causes the center and edges of gradients to be extremely visible since they are not being calculated logarithmically to compensate for how humans perceive brightness/color.
Here is a snapshot of the video I linked above showing what the blur and gradient should look like: