Hence why the 45 degree join to mid downwind as used here in the US is a relatively safe method as everybody is already at circuit height prior to joining - so blind spots below or above is greatly reduced.
They are on an overhead join so long as they are established at circuit height
before entering the "live" part of the circuit - same would apply to the 45 degree US join, ie a/c
should be in level flight before entering the the live part of the circuit.
The way to reduce "blind spots" is to ensure proper lookout - wherever you are.