frishe_nayes

I once spoke to one of the big talis manufacturers about this.
He said that before the war when most of the taleisim were handmade the had dark blue stripes as a zeicher to the techeiles.
Later on when the started producing them commercially they had issues with the blue dye that it became washed out and faded so they made everything black to was more practical.
Since nowdays they are able to use the blue dye without any issues, Belz and many other chasidim renewed the minhag of going with the blue stripes zecher to techeiles.