Techelet is black

Before reading this please note that the Mittler Rebbe is know for giving more than one view point on a concept, flourishing at the end of the opposing viewpoint with a 'de'lo k'nal' - 'NOT like we explained before.'

('ke'nal' - 'as was explained' is the more commonly used expresion.)

However, note also that in the Mamar brought below, that expression is not used.

Also remember that the Mittler Rebbe was a known posek in his own right (and was called "gadol she'be'gedolim" in nigleh by the Rogatchover gaon) and would have every right to pasken on this matter halachlikly.

Also, um, I may have misinterpreted the mamar. Feel free to draw your own conclusions. But remember, you heard it here first! :)

According to the mamar "Beyom Hashmini Shlach" (a shmini Atzeret Mamar) from the Mittler Rebbe it is implied that the color of the techelet on the tzitzit is black, not blue, as is commonly associated.

The reason for this is that techelet is associated with the concept of "yashet choshech Sitro" 

- (Hashem) places darkness around him, a very high level conoting Hashem essential hideness from creation 

(meaning, even if Hashem did not 'hide' himself, he would still be hidden, since he is essentially so.)

It is further likened to the concept of "albish Shamayim Kadrut" - I will encloth the heavens in darkness. 

This, together with the concept brought with it in the mamar that the techelet is compared to the heavens, implys that we are talking about the heavens in the night time.

The heavens at night are black, and not blue like in the day time (and it also makes sense that the sea is not always 'blue either'. In fact it is mostly dark, or even black).

The Mittler Rebbe specificly brings the 'mar'e' shachor' -black color, as an example, in context.

The mamar also compares this to the pasuk "ke'aholei kedar, ke'yeriot shlomo" - like the (black) tents of the Kedar (people), so to the (white) curtains of Shlomo. 

That these two colors techelet and lavan (seemingly black and white) are equaly juxtaposed to each other. 

He also mentions the concept in Zohar Hakadsoh that concerns the colors of the candles. 

'Nehora tichla' the techelet flame in this conext is the same as the 'nehora uchma' - 'the dark flame' i.e. the flame that is closest the fire that burns the wick.

This, because it is the color of the techelet that blinds the eyes of the chitzonim, so it therefore corresponds to that part of the wick. Is it possible that it is the concept of the darkness itself that blinds the chitzonim?
The Mittler Rebbe also equates the nehorah uchma as being the same color as the nehora tichla in the mamar "ve'achar ha'ruach rash".

It is also compared to the Torah being given "esh schora al gabei esh levana" - a black fire on a white fire. Again, we see that black and white are the opposing colors here.

Perhaps this is why it is the chabad custom to place black stripes on the wool talit?

This may mean however, that werever the Torah mentions techelet, such as in the mishkan, we would have to also interpret it as black and not blue.

It is interesting to note that R' Yehonatan Eibshitz considers the tachash to be a zebra, in which case the mishkan would also have these black and white stripes (in addition to the techelet).

Please note that the Shulchan Aruch and the Shulchan Aruch Harav do not cleary state what color the techelet is (I searched every techelet mentioned).