This feature is only available in the desktop editor.
Create target article
Open article with content blocks you want to move
You should now have two articles open.
Option 1: Copy and Paste
With both articles open you can copy select the content blocks you want to copy and use Edit > Copy to copy the content blocks to the clipboard. Now switch to the new article and use Edit > Paste to paste the content blocks.
Option 2: Drag content blocks from one article to another
If you want to copy the content blocks, hold down the OPTION (Mac) or SHIFT (Windows) key while you drag the steps.
The content blocks are moved to the new article. In the screenshot below the content was copied from one document to the other.