Is this dark line under eyes Tyndall effect or hyperpigmentation? (photo)

Hello and thank you for your question. Based on your photo, it seems as though your tear trough area may have been overfilled causing an indentation in the surrounding areas and thus casting a shadow, causing it to look darker in certain lighting.  I would suggest having your injector take a look and he/she could dissolve some of it down if that is the case. Best of luck.

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From the photo, it appears to be a shadow beneath either a swollen or overfilled lower lid.  I recommend you contact your injector and have it assessed for possible hyaluronidase treatment.
On the bright side, there does not to be redness of infection, nor hematoma.  The tyndall effect has  a bluish hue, seen best in sunlight.
I hope that helps!