Are the impurities that cause darkening of the neck, and the oil that comes out of the scalp after scratching the hair, considered a barrier that prevents the validity of ritual bathing (ghusl)?

A tangible barrier that prevents water from reaching the skin is what invalidates the purity. As for non-obstructing dirt, such as a mere discoloration on the body, the ablution is valid with its presence, as is dried sweat. As for grease on the hair, if it is solidified and prevents water from reaching [the hair], it must be removed. Otherwise, the ablution is valid with its presence.