How to send a message
To start a conversation, just look for the speech / chat bubble. 
From the app:
- Tap the chat bubble to open activity and messages.
- Tap the new message bubble.
- Enter a blog name.
- Start a conversation.
From posts:
- Click or tap the share icon.
- Enter the name of a Tumblr you want to send that post to.
- Click on the desired blog to select it.
- Include a message to start a conversation.
From the web:
- Click the chat bubble at the top right of your dashboard.
- Type the name of a Tumblr blog.
- Click on the desired blog to select it.
- Type your message.
To send a picture in a message, click or tap the camera icon, choose an image (including GIFs), and send. Or you can use an existing Tumblr GIF: just click or tap the GIF icon instead, and search for a GIF that you like.
Who you can message
You can message almost anyone. It’s really up to the receiver—they have all the control. If they want to accept messages from everyone on Tumblr, then you can message them. If they only accept messages from Tumblrs they follow, you have to wait until they follow you first.
Keep in mind: You can’t message group blogs(at this time), or anyone who’s blocked you.
Another thing to note is that you might receive a message from Tumblrbot. Tumblrbot is an official Tumblr bot and it might message you (just once) if it thinks it can help with something. Reply with “stop” to stop receiving messages from the bot.
To encourage communication, you can let your Tumblr friends see that you have recently been active on your blog. We indicate this with a green dot next to your avatar. If you'd rather not show this, you can disable the feature.
To prevent people from messaging you, go to your blog settings and enable “Tumblrs you don't follow (except your Post+ supporters) won't be able to start conversations with you.” Now, Tumblrs you don’t follow won’t be able to start conversations with you. (Note: this setting will not apply to users with whom you've previously had a conversation or active Post+ supporters).
Other Settings
Messaging sounds
- In the app: From your Account tab, tap the gear icon, then Settings, and then "Sounds." You may enable or disable messaging sounds from there.
- On the web: From your dashboard settings, you may toggle the "Messaging sounds" option on or off.
Disabling messaging entirely is not possible. However, you can block senders and delete the entire message thread by clicking or tapping the meatballs (three dots) in the top right-hand corner of the conversation.