When converting and image to B&W, blue tones are not converted to grayscale but retaina blue tint which is deepened when adjusting B&W mix

One thing to check for is adjustments made in panels such as Color Grading or any individual RGB channel Tone Curve. Those adjustments are applied after the profile so if they create any color shift, that will shift the B&W version away from neutral. If you find any of those, reset them to neutral.

In the case of Color Grading at least, that’s intentional, because it’s how someone can simulate a tinted monochrome process such as cyanotype, sepia toning, or other types of for B&W split-toning methods.

Does your image have a sky mask that makes the sky more blue? In that case, it may overlay the sky with a blue tint. That effect remains when you change the image to black & white, because it is a color overlay.