On a scanner you sometimes need to be able to move the mirrors without a laser line being drawn. TTL blanking will turn the laser off.
An example, if you project a word in capital letters the blanking can be used to make sure that the letters are not joined together by a line, that part is blanked.
TTL modulation I think means to adjust the intensity of the beam rather than fully blanking it. On RGB laser projectors that would allow you to have a wider colour pallete.
Hope that helps.
Regards rog8811