Firstly, many areas now feature “ dark grass “ or “double grass”, which can be identified by being a slightly darker shade than normal tall grass. Pokemon found in dark grass are usually higher Level than their regular counterparts and often appear in pairs, sparking a double battle.
its a double battle grass, and its a little bit darker than normal grass
the right picture is the dark grass, the left picture is the normal grass
picture from gamerguides.com
so I found this question on this site thats tell what are all of the dark grass location
If you look for route 1, 3, 5, 6, 7, 9, 10, 12, 15, 16, 18, 20, or 23 you will find some dark grass for double battles.
But keep in mind that route 9 will be frozen in bw2 until you get through the main game.
There's also the forests pinwheel forest and lostlorn forest.
Virbank Complex, Castelia City, Abundant Shrine, dreamyard, Cold Storage, Dragonspiral Tower, and Victory Road.
if you are playing W2 thats mean your in luck, reversal mountain have a flame orb in it, but only accesible for W2, while toxic orb is available for B2.
IVE FINALLY FOUND THE ANSWER!!!
When Pokemon hold an item 1% of the time, you can only get that item if you fight it in the dark grass patches. Since Heatmor is only in Twist Mountain, you can't get a Flame Orb this way.
so you cant get a flame orb this way, only by buying it in the battle subway and PWT. (also reversal mountain if you have W2)
Source: heatmor and flame orb
Source: Dark Grass
Source: location of Dark Grass
hope I helped!
