It is not at all a new trend.
It is mostly by Belzer Chassidim, but not limited only to them.
the source is not 100% clear, as it is an ancient minhag dating back so far that it isn’t really clear from when.
It is supposed to symbolise a zecher to techelet, as techelet wasn’t/isn’t around (machaloket poskim), and serves as a reminder of techelet.
That was how they used to make tzitzis all over was with a slight blueish ting, not a strong blue, but a very dark mild blue-almost black.
However, when the so called “religious” zionists started putting blue on their taletim to symbolise their flag, it was widely accepted all over not to go anymore with blue stripes, even as mild as they were, however Belz decided to stay with blue, citing their mesorah.