Assuming ONE_MINUS_SRC_ALPHA / SRC_ALPHA and the default blend formula you should get what you described.
However, if you have the formula reversed (1-a and a swapped), you get:
out = dst*0.9+0*0.1
which leads to:
(5*0.9 = 4.5 \~ 5)
Any alpha value more or equal than 0.5 won't fade to black because the result will eventually get rounded to more than 0.