This drove me nuts for a few minutes, so I figure you might need help too. That probably explains why you had to Google ‘Block Select’ and wound up here.
Here is what you want:
There’s No Trick, But You Do Have to Enable It
The default behavior is to do a normal ‘wrapping’ selection of text. You can disable this under the Edit menu.
Selecting Text In Action
On a newer version of SQL Developer?
A lot has changed since I first wrote this in 2012. Here’s what it looks like today, and probably since about version 4.1/4.2 time frame.