The meter does have its own take on gestalt, in fact--capturing the elements that are part of the crime nexus (what's often called "hard-boiled") with the psychological nexus (what's often called "gothic" or "women's picture" or "melodrama").
UNDERCURRENT is certainly in the grey zone of noir, and it shows up with a decided tilt to those "melodrama" elements. Its "melo rate" is 163, compared to an overall average of 110 ("melodrama" does operate more intensely in noir than the "crime" elements).
I think the film has been unjustly maligned, but having said that it is no masterpiece. These "gothic" films were sort of the steppingstone to "arthouse fare" that would begin to emerge more prominently "on the continent" in the 50s (though of course there were guys like Albert Lewin around as more flamboyant precursors to such a direction). Minnelli finally made a kind of "arthouse" film a number of years later with TWO WEEKS IN ANOTHER TOWN (1962). Oddly, he paints with much more broad a brush in that film, losing a lot of the "sneaky" artiness that he packed into his earlier work (as a perusal of Mike Grost's formalist inventory of Minnelli's motifs will show you).
As incendiary as the ultimate collision of Joan Crawford and Bette Davis was in BABY JANE, it would really have been something if they'd put Hepburn and Davis into the same film. Would that film literally have exploded while been screened, or would they never have even managed to get that far??