Is is possible to save blocks from one mailing to reuse in another mailing using CiviCRM Mosaico?
One way that I  am aware of is to click into a block in one mailing and then select the "Source" button 
 

That will bring up the HTML source code dialog window  

You could copy this, and then paste it into a block on the other mailing and assuming there are no fundamental differences in the template's stylesheet, it should look the same.