If you're working in CMYK mode a 100% K ink is definitely not going to give you a deep black color. It's going to be a dark gray or, as you say, a dark brown. What you want is a "rich black," which means a deep black color made with all four CMYK ink colors. There are various rich black formula combinations of CMYK ink colors with the percentages adding up to a total ink value: the percentage number of all CMYK ink percentages added together. One is C:75, M:68, Y65, K:90, which adds up to 298. That's a good rich black value. Some people like C:30, M:30, Y:30, K:100, but that only adds up to 190. Definitely do not do 100% on all four CMYK values. That would be really bad when output on something like a thermal inkjet printer (we're talking ink literally dripping on the printed substrate). Depending on the printer and its settings, some print devices will detect a one ink 100% K black color in artwork elements and automatically boost it to rich black values. But, generally speaking, it's usually best to apply the correct CMYK color values to the artwork.