Blended malt was previously called pure malt. The malt part of the term, as usual, means that the whisky is made from maltef barley only. No other grains allowed! Blended means that whisky from multiple distilleries has been used. Sometimes those distilleries are diclosed, sometimes they're kept secret.
The perhaps easiest mistake to make is to confuse blended malt whisky with blended whisky. The latter and vastly more common term doesn't consist of only malted barley, but grain whisky as well.