This sounds like dirty paraffin. Dirt-particles can lead to knife marks. It is recognisible, if the marks are also in the "only-paraffin-part" of the block. You can try to filter the paraffin. Melt the block again and embed in clean paraffin.
If the tissue itself has hard particles inside, it may be calcification. Is that possible in your case? broken parts of rat-bone?
Venecian blind is usually caused by some loose part of the microtome. Knifeholder, blockholder, ...
or too fast cutting, vibrations of the knife.
Some tissue is sensible for too long soaking. This can be seen, if the tissue "swells" out of the block-surface.