What is a B Flat Block Flute? Where can I Buy One?
Block flute is a term usually used for recorder. Recorders are usually in C or F.
Where did you hear of this instrument?
John is, of course, accurate. Blockflute usually describes a recorder, particularly in Germany. The whistle (pennywhistle, tinwhistle, irish whistle) is also technically a block flute or fipple flute in contrast to the transverse or side blown flute usually discussed on this forum. The whistle is not fully chromatic as is the recorder. The whistle is designed to play a simple diatonic scale. Many whistle makers produce instruments in Bb. If that is what you are looking for you may want to ask the same question on the whistle forum.
http://chiffboard.mati.ca/viewforum.php?f=1
Generation makes a decent Bb whistle which is quite affordable and readily available in the usual shops.
Hope that helps.
Feadoggie
The correct spelling would be “Blockflöte” and it is indeed the german word for “recorder”. Though I’m not too sure if “block flute” really is ment to be an synonym for “recorder”. When googeling for “block flute” strange looking instruments pop up - it more or less looks like the term “block flute” is used as a family of all woodwind instruments with a fipple.
If this assumption is right a whistle would also classify as a “block flute”.
Wikipedia only knows about “Fipple Flutes” (http://en.wikipedia.org/wiki/Fipple) and if “Block” is a synonym for “Fipple” so is “Block Flute” for “Fipple Flute”.
Yes, they are synonomous. The “block” or “fipple” is the plug in a recorder/whistle mouthpiece which fills the top end of the bore and forms the windway floor. Another term for this genre of instrument would be a “duct flute”.
Feadoggie
By the way there is indeed a “BlockFlute” or Recorder available in the key of Bb. It is made by Kung, or Kueng in the German spelling. I have two Soprano Kungs in C and they are very nice instruments. Antique Sound Workshop carries them as does Lazar’s early instruments. The Bb, or ‘Folklora’ model on the Kueng website is available in Pearwood and also Palisander (rosewood)
Yes, not to be confused with a keyed Bb flute which is block mounted.
(just to make this topic Flute Forum relevant)
Block-heads on the Flute Forum 
You CANNOT be serious! 
Could it have been a Bb Black Flute? I’m serious, that would be an easy typo.
For a block-head.