The 22-year-old ex-England Under-19 keeper has had previous loan spells at Middlesbrough and Ostersunds in Sweden.
He comes in after the Chairboys failed to agree terms to extend the loan of Sheffield Wednesday's Cameron Dawson.
If 6ft 6in Blackman starts against Accrington on Tuesday, he will be the fifth different Chairboys keeper in five consecutive regular season games.
Blackman will follow Benjamin Siegrist versus Accrington, Ryan Allsop against Oxford, Scott Brown at Crawley and Dawson versus Grimsby in playing for the Chairboys in League Two.
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Chelsea goalkeeper Jamal Blackman has joined League Two side Wycombe Wanderers on loan until 3 January.