On the Wikipedia page for Irene of Athens, under the section "Rule As Empress", the second paragraph says "A solar eclipse and darkness lasting 17 days . . . " This was in reference to to Constantine VI's death. The wiki page for Constantine VI makes no mention of the eclipse or the "17 days of darkness".
I was hoping to have someone clarify what I imagine to be some sort of volcanic activity or perhaps an apocryphal tale.
Others would be better equipped to talk about apocryphal or supernatural bits showing up in Byzantine histories around that time (all I know is that they're not uncommon).
As to the eclipses themselves, there were two total eclipses worldwide in the year 797! One occurred on August 26 with totality running along a path from California through Central America and into eastern Brazil, and the second on March 3 across an arc from Argentina, crossing the Atlantic, and terminating in Sudan.
While they would obviously have lasted for an hour and a half at most rather than days from any particular viewpoint, the March one would have been visible in Constantinople. Residents wouldn't see a total eclipse, but just a partial one, obscuring about a fifth of the sun's disk in the early evening (about 6:45 pm as we'd reckon it, to be exact).
(edit: whoops, got them reversed. The March eclipse would have been visible in Constantinople, not the August one.)