I believe you mix black or blue dye with the regular teal dye, and then craft a blanket with it.

Yeaah no, there is no such thing as dye mixing in this game. Dark Teal is a dye, its a winter dye, so your only way of obtaining it now is to buy it from someone

oh. I'm not really a crafter in general so I'd go with what maki said 😅

Dark Teal dye can still be found in explore as well.  Scrap yard and rare.  This seems to be the same for a lot of the seasonal items, as I recently found one of the black cats there as well, Frasier Fir dye, Burned Leaf dye (pretty sure that's the name of that one), etc.

Dark Teal is a seasonal dye and only found during Winter, so not until way later in the year. Teal dye is a Summer seasonal dye that can be found now; while you can find old seasonal stuff in the Scrapyard, those type of quests are usually timed and it's a strong rng chance to even stumble upon Dark Teal dye. Your best bet is to either A) purchase it from someone via the Marketplace Trades or simply B) Ignoring the quests that ask for seasonal dyes, unless it is the proper season. Which Summer dyes, are roughly as follows, Cobalt Blue, Neon Green, Sunburn, Electric Blue, Teal and I believe Sky Blue. 

There is a Google Docs in the Help channel of the HoFa discord that is pinned and it has Seasonal Dye information there.