Records for the Blumers are found in the Lehigh County Pa churches, particularly Zion Reformed church in Allentown and Jordan Reformed church in Whitehall Township as well as other churches that were included in "the Whitehall Charge" of which Abraham Blumer was pastor from 1771 to 1801. They are available on microfilm through the Mormon Family History library. If you're anywhere near Pennsylvania, copies are in the library of the Pennsylvania Historical Society in Philadelphia, the Northampton county Historical Society Library in Easton, the Lehigh County Historical Society in Allentown. Lehigh county Historical Society also has a newspaper file that lists items such as births marriages and deaths that appeared in the German language newspapers in the area. German Reformed church records, both the original German and some translations are at Franklin & Marshall College in Pennsylvania. Some of the churches are still in service, but are now affiliated with the United church of Christ.
Not all of Pastor Blumer's grandchildren's births are in the church records. He may have baptized many of them privately after he retired in 1801. His youngest daughter Susanna is my ancestor and only one of her four children appears in the church records. The girl was christened after Abraham Blumer died. There are two histories that should be available on interlibrary loan through your local library: "History of Lehigh county" 1914 edited by Charles Rhoads Roberts & others, which might be in Fiche form, and "Allentown, 275 years". There are other Allentown histories as well.