Common ground between primary and secondary SMPS supply safe?
That's OK provided that you realise that everything is now live, including the blue wire.



Figure 1. Surprise awaits the unwary.

Even if (1) is connected to neutral, high voltages will appear on (5).

Consider when (2) goes negative. Current will flow through diode (3) and point (4) will be at negative mains voltage + 0.7 V (for the diode drop). (5), the SMPS and DUT will be live.

When (2) goes positive diode (6) will be forward biased holding the blue line (5) at 0.7 V above (1).

Be very careful. Insulate everything. Do not connect oscilloscope probes to DUT (unless the 'scope is a fully insulated type such as the Fluke Scopemeter) as you risk shorting mains to ground through the scope.