Items in blind boxes are unusual as they get sold and bought as one collectible (the bag or box with an unknown item in it) and then when they join your collection you generally know what they are.
Upon unpacking it you can reassign your new Collectible by clicking Actions → Reassign and finding the correct Database Item (see Change/Reassign a Collectible's Database Item for more details):
Your Collectible will now be linked to the right item and will show the correct Estimated Value! If you don't know the name of the unpacked item, you can check the Database Item for the blind box and it should show you all possible items that the box could contain:
TIP: Scanning the blind box's barcode with our hobbyDB Vision app will get you right to that page!