Are blocking diodes necessary for solar panels
They are required.  If the panel becomes shaded it is considered DARK. If the panel is Dirty it is Dark. What happens is your produced voltage has a PATH to the other panel. The Diodes block that action - allowing current to flow in only one direction.


  These blocking diodes, also called a series diode or isolation diode,
  ensure that the electrical current only flows in one direction “OUT”
  of the series array to the external load, controller or batteries.
  
  The reason for this is to prevent the current generated by the other
  parallel connected PV panels in the same array flowing back through a
  weaker (shaded) network and also to prevent the fully charged
  batteries from discharging or draining back through the array at
  night. So when multiple solar panels are connected in parallel,
  blocking diodes should be used in each parallel connected branch.


 Excerpt above from here ^

Connecting Solar Panels / Cells