While waiting for a next update of Blocs, here is a tip to find the “background” effect as in Blocks 3.
You have to modify a value in the “universal-parallax.js” file once the export is done.
Open the file with “Textedit” for example.
Look at the last line and change the value to 1000 after “speed:”
Initially this value is 3 (this parameter manages the scroll speed when parallax).