BAD GRAMMAR WARNING.
It's a extremely rare red block that does nothing but it will run forever, like an forever C block without nothing in, on scratch online/offline editor will look like a rectangle (do not get confused with stack blocks), to get the block go to this proyect , it will look like this: 
Variable with no text in
it's a variable that does says nothing, it's form will look like a curvated small at Y size box, to get it is editing .json from .sb2 file
custom Short block size
It's a custom block that does says nothing, it's form is short, you can't create a custom block without name or inputs, to get a short block is editing .json from the .sb2 file