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 try other piperine containing types of peppers (Piper genus), these include P. longum (long pepper) and P. retrofractum, as well as white pepper P. nigrum, though any member of the Piper genus should contain some piperine, but amounts and hence spiciness will vary.
You could also try ginger (Zingiber genus), as well as mustard seeds and shoots/leaves (Sinapsis genus and Brassica juncea), all of which are objectively "hot", though also with different (and strong) flavours and heat levels.
Wikipedia also has a list of peppers under their Template:Herbs and Spices, which includes such things as Alligator pepper (Aframomum), Schinus genus, Sichuan pepper (Zanthoxylum), and a couple of others.
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.
If you're looking for something to mimic the flavor profile of chili peppers, to give you familiar-ish tastes in your food, the best I've found is sweet or smoked sweet paprika.
Sweet paprika is made from bell peppers would contain no capsaicin, since bell peppers do not. Hot paprika, on the other hand, is made from capsaicin bearing peppers and would not be safe - and some paprikas are made from mixed batches of peppers, so it may take some label-reading and deciphering to make sure you get a pure-bell-pepper paprika. I've only seen smoked sweet paprika in my area, though it seems that smoked hot paprika is available elsewhere.
I'm pretty strongly intolerant of capsiacin, and this works for me to get the flavors in food balanced well when working with recipes that call for chili peppers. It will of course still be different, but it can be flavorsome and very good. I would suggest caution, though, when trying paprika at first as tolerance levels can vary a lot - mine is severe enough that I consider sweet paprika safe enough for me as safe to recommend, but there's always the possibility of individual differences.
If what you're missing is not flavor-profile-stuff but the actual spiciness, bob1's answer on foods and flavorings with other spicy compounds may work well.
Or you could maybe just try eating your food while really boiling hot, to replicate the scorching of one's mouth :)