Does it take awhile for a slight concussion to the back of the head to heal?

Yes. A concussion is a "bruise" to the brain. Depending upon the severity and if there is any brain swelling or pressure change in the skull, symptoms can vary widely. It's a good idea to stay in contact with your doctor or neurologist after a head injury to watch for new symptoms but it may take weeks to recover.
Concussion. It may take weeks or months for the head to heal fully after a concussion. Once you have had one concussion, it is easier to have another.