France Sc#87 Prussian Blue?

While, as we know, it's difficult to determine colors through the screen, I'd like to show this example that I think could be an SC#87 Prussian Blue, since it might be possible to determine it without resorting to color... Although at first I thought it would be cobalt blue, a new observation leads me to think of Prussian Blue:
-The postmark date could coincide with the timeframe of the issue
-The rounded lower left corner
-The blue-green lines on the reverse

Posted some time ago by GeoffHa

https://www.spink.com/lot/18054000117

thanks, shame not being able to see the back...

Aside from the colour, your example is also postmarked six years after the Prussian blue was issued.

Yes, the cancellation of the 86 is what led me to think of a cobalt blue, but today I read about the Prussian blue, " the frame line is partially transparent on the reverse". That, along with the rounded corner, is what makes me doubt my stamp... anyway, since it's older than 80, it can't be ruled out.

I think we can safely rule yours out as being Prussian blue Murasama. Sorry to disappoint, but it is what it is

Any tricks to determine this with such certainty without relying on color? I have it classified as cobalt... but the partially marked frame lines on the back make me doubt...
"Autre "truc" d'expert: lorsqu'un timbre est un bleu de Prusse, il arrive fréquemment que la ligne d'encadrement transparaisse partiellement au verso..."
I found this certified example... with the frame line on the back... it's obvious that it doesn't match mine. It's the second example I've seen, and the line is always clean and fine, not blurry like mine... So I'll continue to maintain its initial classification of cobalt blue, the most logical considering the type and date of cancellation...