=AVERAGE(J5:J220)
should work whether they are "truly blank" (empty?) or not.
If they only appear blank, they probably have some kind of text (null string; string of spaces), which is ignored by AVERAGE.
However, if the average is incorrect, consider the less likely possibility that the data is numeric, but it appears blank due to formatting or conditional formatting.
Select each apparently blank cell individually. What appears in the Formula Bar?