The Boolean data type is used to express logical conditions. For example, a calculated column defined by the following expression is of Boolean type:

= Sales[Unit Price] > Sales[Unit Cost]

You will also see Boolean data types as numbers where TRUE equals 1 and FALSE equals 0. This sometimes proves useful for sorting purposes because TRUE > FALSE.

The Boolean data type is nullable and can store BLANK values.
Comparison operators return only TRUE or FALSE instead of preserving BLANK values, with one exception. There is an optimization when a boolean expression is compared to constant TRUE: the DAX engine will simply return the boolean expression directly bypassing operator= completely. There is no optimization when a boolean expression is compared to constant FALSE, so operator= is executed.
If you just want to get rid of the BLANK values in a boolean expression, you can use COALESCE( …, FALSE).