With tools like the oil brush and water color you can do something like that, yes. You'll need to turn off the "Auto Clean" option on the Settings panel for each of the tools. This will place a water cup at the lower right of your screen (to "clean" the brush). So, when you drag the brush through other wet paint, the brush will pick up that paint and blend it. The brush will retain that blended color upon making another stroke, unless you clean the brush the cup (which resets your color back to the last color you selected in the color picker), resize the tool (as AR sees that as you selecting a new brush), or continue painting strokes onto empty canvas so much, that the picked up color fades out (again, leaving you with the color you had last selected with the color picker). This will also allow you to make dual color strokes, by dragging one half of the brush through wet paint, while leaving the other half drawing on empty canvas. More on this can be read HERE along with a visual example.
On the other hand, if you just blend one color into another and want the blended color of the two, you can just hold "Alt" and left click over the color to pick it up.
Nothing is easy to the unwilling.