In such cases, it may also depend on the type of residence permit and job you had before the issue date of the Blue Card. In similar situation, I got an answer from Auslaenderbehoerde that the 21 month can only be counted from the issue date of the Blue Card. On the other hand, I know an example, when a friend of mine got it counted from earlier date.
For example, in Berlin, Auslaenderbehoerde counts all the months when the applicant had a job that satisfied Blue Card requirements, literally "... you must have been in employment for at least 33 months.
That employment must meet the requirements for the issue of an EU Blue Card."
Also, if one studied long enough (during the time with residence permit for the purpose of study every 2 years in Germany count as 1) and paid pension insurance, he/she might be eligible for Niederlassungserlaubnis even earlier than after 21 months (5 years living in Germany and paying pension insurance are required to get Niederlassungserlaubnis according to §9 AufenthG or Daueraufenthalt-EU according to §9a AufenthG, which is another regulation applicable). However, usually this is not the case and Blue Card holders get Niederlassungserlaubnnis after 21-33 months according to §19a AufenthG. After that it is still possible to apply for Daueraufenthalt-EU later.