Asks are a method of both private and public communication on Tumblr. They can be sent to whomever has Asks enabled, and your message can be sent anonymously if your recipient has additionally opted to enable that feature. Anonymous asks must be answered publicly, and cannot be traced back to the sender even if the sender was logged in to his Tumblr account.
Open the Ask window for the blogger to whom you want to send a message. You can open the window in two ways. Hover over his user icon on your dash, click the white user icon next to the "Unfollow" button and select "Ask a question." Alternatively, visit his blog's Ask page directly. Access all Ask pages using the same URL if Asks are enabled: http://xxxxx.tumblr.com/ask.
Type the message you want to send into the text box. If you aren't logged in, the message is sent as anonymous by default. If anonymous Asks are disabled for this user, you won't be able to send him a message. If you are logged in, check the "Ask anonymously" box. Note that if a user has disabled Asks, you cannot send him one whether you are logged in or not.
Click the blue "Ask" button to send the Ask to the user.