Ideally the blade for a rotary mower is sharpened to a 30 degree bevel
and then blunted by about 1/64" because a razor edge is not durable.
In manufacturing if the blades are milled or stamped or milled stamped
and hardened the edge burns away to about what you saw on the new
blade.

Jen wrote:

To balance a mower blade,

Surely it would be much safer just to buy new ones. As I said new ones
*seem* blunt but cut well. Obviously they are sharpened the way they should
be.

Jen