Are the black spots running vertical with the nail, or are they actually circular or irregular circle 'spots'? If they are vertical, it could be splinter hemoraghes. These usually occur due to some sort of trauma and are the result of voids in the nail plate cells which fill with blood. As they reach the free edge, the nail plate layers can split laterally, but usually do not split vetically. By 'laterally', I mean that the nail plate layers can separate in the area of the void.
Do you wear shoes that are tapered at the toe? The more taper to the toe of the shoe, the more stress placed on the big toenail - not only from 'sideways' pressure, but from 'downward' pressure. Try wearing a different type of shoe - one with a wider or more rounded toe and not so tapered.
As for the ridges in your fingernails, these come from age. As we age, the nail bed does not secrete the normal 'tidal flow' of oil and moisture that transmits up through the nail plate layers to keep the nail plate flexible and strong. The ridges you see are actualy the 'rails and grooves' upon which the nail plate rides forward on the nail bed - much like a train rides forward on its tracks. You can replenish the levels of oils and moisutre by twice daily applications of a good quality nail and cuticle oil that contains (among other essential oils) jojoba oil and vitamin E oil. I recommend Solar Oil which is available in fine nail salons or through Ulta Cosmetic outlets nationwide, or Botanical Oil which is available in the nail department of Sally Beauty Supplies.
For more information on your natural nails, please visit my (approved) website: www.hooked-on-nails.com