Reference for Niddah 37:51

וממאי דהני טהורין והני טמאין

So it was also taught: Black blood is like the sediment of ink; if it is dark it is unclean, and if lighter, even though it has the colour of stibium, it is clean. And black blood is not black originally. It<span class="x" onmousemove="('comment',' Being originally red. ');">⁴⁶</span>

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