The color picker can work with HSB or RGB color models. These are three-dimensional models, so they’re a little tricky to represent in 2D. The bar in the color picker follows your color model / channel selection, and the rectangle shows possible values given the selection represented by the bar.
When you select BLUE (as in your picture), the bar shows the possible range of blues, and the rectangular area plots red against green for that specific blue value.
When you select HUE, the bar shows the range of possible hues, and the rectangular area plots saturation against brightness for that specific hue value (which is perhaps what you are expecting?).