Okay, I have replaced the RGB and the alpha channels of detaile_nm.dds with plain grey color, and the problem persists. At least we can exclude tha file from the list of suspects.
Something that I have noticed is that the visual glich only appears along step shores, where the sand texture gets blended with the texture used for slopes, the one combining diffuse and normal maps. That might confirm my impression that the blue spots are actually abnormal light reflections. Now that I think about it, there is something else that I have noticed in the past days: the darker the sand texture, the more obvious is the bug; at some point, while making experiments with an especially dark texture, the glitch was so severe that it looked like lens flares...
My guess is that, when rocky slopes and shores mix together, not only cliff's diffuse texture gets multiplied by the color values of the diffuse shore texture (what is desiderable), but also the normal texture gets similarly multiplied, thus causing abnormal lighting effects. That coul explain why the bug is not noticeable with light sand tectures, as in image computing a colour multiplied by white is equal to the same color (white pixels are equal to 1's). If that was true, the only solution would be giving up dark sand textures or finding the mistake in the game shaders
