There are two parts to the forearm - the inner forearm, which, when your arm is hanging relaxed at your side, is the side closer to your body, the side your thumb is on, and the outer forearm, which, when your arm is hanging relaxed at your side, is the side farther from your body, the side your pinkie is on. Thus, a block with the inner forearm as the tool is an inner forearm block, and a block with the outer forearm as the tool is an outer forearm block.
If, as I suspect, you are referring to the direction of the block, rather than the tool, then there is another term that must be added - inward or outward, which refer to the direction the block travels relative to the centerline of the body. For example, an inward outer forearm block is a block that moves from outside the shoulderline toward the centerline, using the outer forearm as the tool; in contrast, an inward inner forearm block is a block that moves from outside the shoulderline toward the centerline, using the inner forearm as the tool. An outward outer forearm (often simply called an outer forearm block) would move from the centerline (or sometimes from the opposite shoulderline) toward the shoulderline of the blocking arm, using the outer forearm as the tool; an outward inner forearm block (often simply called an inner forearm block) would move from the centerline (or sometimes from the opposite shoulderline) toward the shoulderline of the blocking arm.
I hope that makes sense - here are some visuals:
In Chon-Ji, the blocks in the first half of the pattern (moves 1, 3, 5, and 7) are outward outer forearm blocks, while the blocks in the second half of the pattern (moves 9, 11, 13, and 15) are inward inner forearm blocks - although the terms "outward" and "inward" are not generally used in describing these blocks.
[yt]CRRUvtVXWLE[/yt]
In Choong-Mu, the first half of move 24 shows an inward outer forearm block (between the double inner forearm block, with the right fist to the left elbow and the backfist) - it's hard to see because his back is to you, but you can see the direction of the motion.
[yt]HxbB2Wiv2R0[/yt]
An example of the inward inner forearm block is escaping me at the moment, but if I think of one, I'll post it later.