You are talking about Parallel Import, I believe. This is not a "black import."
Apple, of course, can block devices without new OS. There are many such cases. For example, an iPhone owner can lock their device if it has been stolen. Apple also locks devices stolen from markets. In terms of why Apple won't do this, there are many possible reasons.
Loss of market. Russia is a big market for Apple. Therefore, it can formally follow the sanctions, but still continue to sell products. In addition, blocking devices could affect other markets, such as China.
Politics changes, but costs do not. Therefore, this action will be a burnt bridge for Apple.
Technical problems. How can you tell if a phone is Russian? By geolocation? But that could affect a non-Russian citizen who currently lives in Russia. By language? But many other countries use Russian, including Ukraine.
Blocked devices are useless, but unblocked ones can collect useful information. I wouldn't be surprised if iPhones were used for espionage purposes.