Database design problem: where to put UNIQUE constraint
Since orderInForm is the position of a particular block on a particular form, it should be a column in the FormBlock table, not a column in the Block table.  If orderInForm is in the Block table, a block would need to have the same position in every form it was on.

Once you move the column to FormBlock, you can then create a unique constraint on the combination of (form_id,orderInForm) on the FormBlock table.