How to selectively decrease blue saturation?

Using 'Masking' with any adjustment tool will be a way to selectively de-saturate an area.

Using 'Color Masking' and the dropper you can select the color to desaturate (blue).

Using 'Luminance masking' you can define (by narrowing the pointers) the tone brightness areas to affect.

If the walls are white, then a 'Luminance' masking can limit the desaturation only to highlight/white areas of the image.

A hint to select the total image and apply selective masking is to use a Gradient that covers 100% of the image. (Start outside the image and drag away from the image.)

Even with a Gradient active you can fine-tune a selection as the 'Erase' brush in the Gradient tool can remove sections of the gradient effect.

https://digital-photography-school.com/range-mask-lightroom-classic-cc/

https://helpx.adobe.com/au/lightroom-classic/help/apply-local-adjustments.html#local-adjustments-color-luminance-depth

For a white surface with a blue tint you could also try a brush with a negative saturation setting.

Is this a Lightroom Classic question?

The only way you can change color selectively is by using brushing (or other local adjustment) and modifying the Tint or Temperature, or by brushing using the color selection selector. There is no such things as the HSL/Luminence/Saturation sliders when brushing. (But you could do this in Photoshop)

Since you don't provide an example, I would imagine that the Temperature selector might help, but that's pretty much a guess; it would not help in all situations.