select all blacks in blue channel?

[Weekend]

hi I'm doing some texture work and I need to select all blacks in my blue channel but I can't find a good way for it without changing other channels.

in Photoshop it was supper easy (go to blue channel and select color range) but I can't wrap my head around it in affinity photo. I select all blacks in blue but when I change them all other channels get affected somehow?!

Here is a sample:

 

[Weekend]

if you turn to blue channel, there are blacks that I want to change them to gray or white

[Weekend]

oh select sample color worked this time! (did not worked last time! )