To insert a blank line below the current line use `:put` or `:put!` to insert one above.

To specify where to insert the blank line (<i>n</i>th line) use `:[n]put`.

e.g.1: Add a blank line below line number 23 while in command mode.

 :23put

e.g.2: Add 5 blank lines above line number 23 while in command mode.
 
 :23put! =repeat(\"\n\",5)

Some sincere notes and warnings:
- This answer is an improvement upon that of @Mr Shunz and @justerman, who provided some superfluous information.
- @Mr Shunz's answer has a caveat (flaw). If you use the double quotes as in `:put ="` then you will output a line using the previous expression. It is fine if you used a blank expression previously, but if, for example, you enter these commands in succession `:put =test` and `:put ="` then you will output duplicate lines:
 
 ````
 test 
 test
 ````
- For a single empty line you do not need to use the `=` feature but if one feels compelled to use it then type `:put =''` which are two single quotes.
- I am so surprised at the poor answers listed here! @Akhilesh Siddhanti's answer is incorrect. Do not use it as it deletes a character! 
- I would strongly discourage using a keymap so that you can be proficient in the `put` command. Vim designers have already created such a simple and useful command. You can use `:pu` as a shortcut command.
- @Ingo Karkat may have a script to download that works but I discourage people from downloading a script or plugin unless it is necessary. In this case using a script is overkill. Vim is a powerful, complete, magnificent and simple (to use?) system. You rarely need to add to it.
- @Glorytoad listed an answer using `o[Esc]` which puts Vi into edit mode. This is contrary to the OP (original post) which states "I want to stay in command mode". @Steven Pritchard's answer using `:normal o` is a good alternative.
- Vim version 7.2 was used in my testing.