Confirmed, I can block a random non-existing user through the UI:

mwc.apiRequest(action = 'block', token = mwc.getEditToken(), user = 'pwokwpfok', reason = 'Test bug 38633')

{u'block': {u'id': u'1', u'reason': u'Test bug 38633', u'userID': 0, u'user': u'pwokwpfok', u'expiry': u'infinite'}}

(In reply to comment #0)

1st issue. Could block to non-existing user via API.

recently mediawiki, when execute block to non-existing user via API, it was
blocked as user id "0 ".

In case of previous version mediawiki, doesn't block it, then 'nosuchuser '
error returned.

http://test.wikipedia.org/wiki/Special:Log/
block?page=User%3AHoshiryuhoshiTest

Gerrit change 39860
Can you make a new bug for your second issue please?

(In reply to comment #0)

2nd issue. In case of executing block to already blocked user without
"reblock
arg" via API, In previous version mediawiki, existing block settings are not
changed to new block settings.
but recently when executing block to already blocked user without "reblock
arg", existing block settings are changed to new block settings.

Split into bug 43634.

(In reply to comment #2)

Gerrit change #39860

Merged by IAlex.