Sometimes words with Hs in them were spelled with CH in medieval Latin, e.g., MICHI for MIHI or NICHIL for NIHIL. My own WDI is that the orthography was to explicitly indicate a pronounced H rather than a silent one.

NIHIL OBSTAT is also the motto of the National Registry of Armigers, the second amendment rights organization that believes the purpose of the second amendment to the US Constitution is to give individuals the right to have what are loosely known as coats of arms.