I'm looking for the first appearance of the X-Men jet that looked similar to the SR-71 Blackbird. I already tried googling for it, but it seems to be a very polluted keyword.
It appears to be from X-Men #104, published in April 1977, according to the Marvel Wikia.
It is, in fact, an SR-71 Blackbird. The Wikia page for the X-Men Blackbird:
Professor Charles Xavier somehow gained at least one working version of the RS-150 Blackbird from S.H.I.E.L.D., after the X-Men's Stratojet had been destroyed by Count Nefaria...At some time, the Professor upgraded to the SR-71 Blackbird.
From Wikipedia's page:
When the series resumed in 1975, the X-Men flew a strato-Jet that resembled a larger version of the Lockheed SR-71 "Blackbird" spy plane (hence the name), modified to carry several passengers and for Vertical Take-Off and Landing (VTOL). Some writers have referred to this design as the "SR-73" , "SR-77", or the "SR-70"...
Wikipedia sites "X-Men '92 #1" from May 2015 on Infinite Comics as calling the plane an SR model. It is also known as the "X-Jet."