How can I recover a deleted Messages conversation from a time machine back up of my MBP?

In finder go to...

/Users/YourUserName/Library/Containers/

Open Time Machine & Restore...

com.apple.iChat

Apple hid the Users' Library folders...

Method 1:

1 From the Finder, select the Go menu at top of the screen, and choose Go to Folder.

2 In the window that opens, enter ~/Library, and click Go.

Method 2:

1 Go to the Finder (or desktop).

2 Hold the Option key on your keyboard, and click the Go menu at the top of the screen.

3 With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.

4 Select Library from the Go menu (while holding down Option) to access the hidden folder.

Method 3:

1 Click anywhere on the desktop so Finder is the name of the foreground app shown at the upper left.

2 Press Command+Shift+H

3 With your home folder open, press Command+J. This the only place you'll see a check box to Show Library Folder. Turn it on.

4 From now on, your user account Library folder will be visible without having to use special keys or commands to get there.

Great news, thanks & good luck. :)