Capture One Pro darkening RAW files on import & Selection?

There's no such thing as a "pure RAW file" that you can see on your computer's screen as a viewable image. Ditto for your camera's rear LCD screen. What you see on the camera's screen is a JPEG preview generated using the in-camera settings at the time the image was taken. What you see on your computer screen is one of several possibilities, but in all cases the raw data has to be processed significantly to create a viewable image.

Raw image files contain enough data to create a near infinite number of interpretations of that data that will fit in an 8-bit jpeg file.¹ Anytime you open a raw file and look at it on your screen, you are not viewing "THE raw file." You are viewing one among countless possible interpretations of the data in the raw file. The raw data itself contains a single (monochrome) brightness value measure by each pixel well. With Bayer masked camera sensors (the vast majority of color digital cameras use Bayer filters) each pixel well has a color filter in front of it that is either red, green, or blue.² For a more complete discussion of how we get color information out of the single brightness values measured at each pixel well, please see RAW files store 3 colors per pixel, or only one?

How the image you see on your monitor when you open a raw file will look is determined by how the application you used to open the file chose to interpret the raw data in the file to produce a viewable image. Each application has its own set of default parameters that determine how the raw data is processed. One of the most significant parameters is how the white balance that is used to convert the raw data is selected. Most applications have many different sets of parameters that can be selected by the user, who is then free to alter individual settings within the set of instructions used to initially interpret the data in the raw file.

What you are initially seeing is the JPEG preview image that is attached to the raw file. Once Capture One Pro has time to apply it's own instructions to the data in the raw file, it changes both the thumbnail and the preview window to show the result of processing the raw image data with the instruction set which you've told it to use to open your images. This display of the preview JPEG until the raw data can be processed is common in many raw processing applications.

The reason your raw images look different when you open them with Capture One Pro is that whatever default parameters are being used by Capture One Pro differ from your in-camera settings at the time you shoot the images. You should be able to modify the default parameters to more closely resemble the way your camera is interpreting the information in the raw files.

_{¹ Sure, you could take a picture that contains a single pure color within the entire field of view. but most photos contain a wide variation of hues, tints, and brightness levels.}

_{² Except the "red" filter is really more of a yellow-orange color, the "green" filter is more a yellowish-green color, and the "blue" filter is a violet-tinted blue color. In other words, the colors of the filters in a Bayer mask do not correspond to the three colors our RGB monitors emit and blend to reproduce the response in our retinas that many other colors do. In fact, the colors of the filters in a Bayer mask are much closer to the three colors that each of the three types of cones in our retinas are most sensitive to than they are to the three "primary" colors we use for our RGB color reproduction systems.}