That's just a lot of wear or it might be a grease struck through error, which is when grease gets in the way when the coin is struck, and the details get smeared there. Let's see what the others say.
@Dowell jerry L Yes, it's the result of a grease-filled die. As coins are struck, grease and debris can fill the recesses of the die. With every successive strike, it is compacted and continues to fill the letters, numbers, etc. until they eventually disappear. On your dime, you can also see that "I" of "IN" is partially filled as is the second "6" of the date. It's a common occurrence and doesn't usually add any premium to the value. Chris
