"Not a heatmap" is true and confirmed by FrontierMaybe I dreamed it, but I have a strong suspicion that I saw a "Dev Post" which confirmed that it truly isn't a heat-map showing what we might have hoped it would show...
... but that's not the same as ...
"The colour is completely meaningless", which is false and not something they said either.
It represents some underlying attribute - most clearly visible on the rare rocky ice worlds with biologicals which have bios of both "rocky" and "icy" types and terrain. It's not purely the terrain being represented, either, since some bios on the same planet will have different shades of blue for the same terrain. What the attribute is and how it relates to actual likelihoods of appearance everyone is too busy saying "not a heatmap" to investigate, of course.
On one of those rocky ice worlds [1] I saw blue appear for Fonticula in both rocky and icy terrain. Naturally, the Fonticula only actually appeared in the icy terrain - cutting off very abruptly at its edge, which lined up exactly with a transition from one blue shade to the other, and the rocky bios then started appearing.
So it's certainly not necessarily the case that the flat teal blue is always the best blue to search - though for a lot of bios it's perfectly good! - but it is certainly the case that if you're wandering around for ages and not finding a single sample, trying a patch of a different blue might suddenly find hundreds.
[1] This was a while ago, so "somewhere near Colonia" is as specific as I can be about where. Sorry. I'm sure there are good examples elsewhere, though!