Is this a standard representation of a 'space' character?
The symbol ␢ has a unicode representation: 2422-unicode-blank-symbol.

It is an old typographic symbol for specifying explicit blanks in code.
As Wikipedia states: 


  This was used in the early years of computer programming when writing
  on coding forms. Keypunch operators immediately recognized the symbol
  as an 'explicit space'.


ps. I also learned this symbol as Turing tape blank. I will check an old book later. Nowadays I would use B or if I am feeling adventurous ␣ 'open box' (U+2423) $\text{\textvisiblespace}$.