In my opinion the effect of that kind of Hide in Plain Sight allows you to essentially "thicken" shadows to become impenetrable to vision (whether normal vision or darkvision). However, blindsight doesn't use vision to detect something.
To put it in the terms used by the Hide in Plain Sight ability, vision = observation, but blindsight /= observation, just "detection". For a real world example of the difference, if I touch a teapot and it is very hot to the touch, or if I hear the teapot whistling, I have detected that the water inside is boiling. But I have not actually observed it boiling, since observation would come with me opening the lid and seeing it bubbling.