Yes, you very well can. If you are reasonable about dark malt content then the pH will fall in the right range and everything will be kosher. If, OTOH, you are heavy handed with dark malts the pH will drop low. I've never had this happen (but I don't brew many dark beers except the occasional bock and Irish stout) but people report that the beer tastes thin and sour. It is an easy matter to add some alkalinity to a mash if you do wish to try a brew with large amounts of dark malt. It is almost essential that you use a pH meter to check on mash pH if you do this.