I recently found out that I'm allergic to capsaicin, so that means no spicy Mexican food, which I love.  Are there any substitutions that I can use? I can have black pepper, but too much black pepper and that's all you taste. 
You could ramp up the heat by adding more cumin rather than more black pepper. It's commonly used in both Indian, and in Mexican cuisine, so the flavour shouldn't be too strange. It can make spicy dishes feel much more fiery.  Other possibilities are to use other warm spices, such as cloves, cinnamon, ginger, Szechuan pepper corn, mustard seeds, etc . . . but then again, these might affect the flavour in ways that make the dish taste more Asian, rather than Mexican. 