Looks like either:
1: The insufficient amperage thing. Strange that only the blue channel seems to have dropped, but who knows what the systemic effects of low internal voltage might be.
2: The blue signal has disappeared from the AV port. Was RGB added to your console? I'm only familiar with NTSC SMS2 units where RF was the only output. You'd have to open it up and trace the signal from the connector to the RGB encoder to the VDP. This is contingent that you have positively ruled out cable and/or TV defects.
If you have an connection other than RGB available, you could rule out the RGB port itself by trying it out (composite or RF).
It's probably worth trying a higher amperage 9V power supply first. However, if that same adapter works with both your Mega Drive II & SNES, I'm leaning towards #2.
OEM transformers for NTSC SMS units supplied 1.2A, but I would imagine that the SMS II can work with less. I've used 1A supplies without trouble.
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