yep, with CYA 70, it takes a high shock level, especially with black and/or mustard algae (I think mustard is worse!)
You need to maintain the shock level as consistently as possible- during the day, as Carl said, the FC drops low enough to let the algae grow back.
add more bleach in the AM, to get the FC back up, and whenever you can during the day test and raise FC again.
As aylad said, you can swim at higher levels, even 20-25, with that level of CYA; but might be harsh on swimsuits!
You need to maintain these FC levels until your pool holds FC overnight- lose no more than 1ppm FC between sundown and sunup, without adding more chlorine!