Because HeartGold is poop ^___^
Oh yeah that, Go to Cheats, it should be on second menu. Click New, put this in the first box 020DD9E4 E1A00000 then tick RAW and tick Action Replay DS then name the cheat in the second box Black Screen Fix or something. Click Ok and it'll add the cheat, tick the cheat to turn it on and now it should be fixed. You'll have to tick it every time you play though D:
Oh okay, don't do it if you don't have time =)