John doesn't know about the blades, or the needles. If he did then they wouldn't be there anymore, would be long gone. Destroyed so as to remove any possibility of being found again.

(Sherlock's not about to tell him of their existence.)

These cuts are not a cry for help or a death wish. Simply a coping mechanism, a way to deal with the numbness, the inadequacy and the desolation. A small nick when it's all too much, a drop of blood welling up, and that's enough. Serves as a release, an easing of the tension.
He never scars, never cuts deep enough to scar, always careful to ensure his skin will be unblemished when it heals. He doesn't need any more scars, after all.

He once managed two years without it, but those were special circumstances. On the run fighting for his life, tearing down Moriarty's network, fighting to keep John safe. (Above all else, John must always be safe.) That changed when he came back, saw he wasn't needed now, wasn't good enough, couldn't do anything right. (Couldn't make John happy, replaced by Mary, couldn't be a proper best man, too complicated by feelings, couldn't switch the deductions off for a day, and didn't that one hurt the most? Replaced again.)

Of course John doesn't know about this. And what this is, Sherlock isn't even sure how to define. Normally so verbose, he doesn't have the words to explain these things, these emotions and silent agonies borne out by bloodied skin and screeching violin wails.

(It's not as if it's dangerous. He's had worse injuries from experiments gone wrong. So why should this be a problem? As long as he's careful with the blade or the prick of the needle – always a clean needle, after all, and it's not as if he's injecting anything – then why should anyone be worried?)

(John would worry, if he knew, but he doesn't understand. No one understands.)