I'm inexperienced, so take my advice with a grain of salt. There are only two reported dot com sales on NameBio that are in $xx,xxx range which include "blockchain" and those (theblockchain.com & blockchaintechnology.com) are better in my opinion (although the technology one is kind of ugly - def overpaid in my opinion). All sales on NameBio use the singular version of the word except one (smartblockchains.com) which sold for $147. Of course NameBio isn't including private sales, but I think it's a good indicator. Also, I spend quite a bit of time reading about cryptocurrencies and while someone can definitely make a website on altblockchains.com, I can say confidently that it is not a term used frequently within the community like altcoin is.
All in all, I would try to get $1,000 or so while the field is hot - you might be able to get higher because of this - but I wouldn't be too greedy unless you get a surprising number of offers. My two cents. Key takeaway is don't be greedy because if the field dies down again in mainstream popularity (like it did after the 2013 boom), the prices on "crypto," "block," "blockchain," "coin" and various other types of crypto domain names will likely go down in value.