Black/blue discoloration and swelling can due to hematoma (collection of blood) and injury to blood vessels in the thumb. The swelling may be the reason you are unable to bend the thumb as it can obstruct joint movements. If you are having pain, fracture is always a possibility and it is important to rule it out. But, you cannot be sure unless examined by x-ray images. If you are feeling numbness (loss of sensation) over thumb, there is also possibility of nerve injury. Both blood vessel and nerve injuries are common in fractures. If left untreated, this can lead to extensive tissue damage and possible infection and gangrene due to lack of proper blood supply. In such a case you may lose your thumb.
When diagnosed early, thumb fractures can be successfully treated by applying cast and immobilizing the thumb for few weeks by which time the fracture heals. Diagnosis by x-ray is easy and simple. It will save you a lot of trouble and tension in future if you go now and have a consult with orthopedic surgeon.
Hope this helps. Get back to me if you have any further queries. Thank you.