There are no hard and fast rules, but black pen is probably preferable because it shows up better if the form needs to be photocopied. Check on the form though as it may state a preference. Write as clearly as you can and so that may mean using writing that is not joined up. Do not use capital letters as this indicated shouting on a phone, device or computer and so may imply the same to the person reading the application form. Underline with a ruler if you cannot draw a straight line.