Why do “blossoms” happen?

A backwash or “blossom” occur when the moisture level is uneven.

A weaker mixture of paint is applied on (or next to) a still wet (or damp) paint that is of a stronger mixture of pigment.

How to fix or control a bloom:

To fix a bloom - catch the paint while it is very wet and before you can see it, try dropping a heavier mixture into what you’ve just applied and gently mix it around and tilt the paper. if you see that a bloom has already started to form, dry it. Next, use a stiff brush or a damp sponge, gently dab away the dark paint ridge to match the paint around it. if you can take the whole area all the way back to white, do so. It will then be easier to just dry the paper and reapply the paint. if not, remove what you can, dry it, and carefully apply new paint of the desired intensity.