I drink coffee on a rare occasion (once or twice a month) but i drink teas nearly every day, so yes you can defiantly drink a strong black tea if you enjoy it
did you get the headache after abstaining from coffee/black tea? or did you get the headache when you reintroduced it? if you got it from abstaining thats just your withdrawal from the caffeine, it works much like a smokers withdrawals from smoking. and if you got it when you reintroduced it thats your body "yelling" at you for reintroducing something that It wasn't used to having anymore. Listen to your body, if you feel like you absolutely "need" your morning coffee then go ahead and have it. I allow myself a space for 1 "cheat" each day, that way I stay on track because its like a reward for being a healthy eater for the rest of the day

also, if you have a day or two each week where you find yourself eating one of more unpaleo foods try to limit yourself to just those 1-2 days, any more will probably make you go back to your old eating habits. I know that I will have grains every now and then, but when i do I make sure that its only once that week. the key is to be flexible and forgive yourself if you give into temptation. no need to beat yourself up
