I have a couple of suggestions:
First, to get correct standard errors with either the svyttest() or the svyby() followed by the Census tool, you should be using replicate weights. The way to incorporate these in R using the survey package is as follows (based on this thread):
svy <- svrepdesign(data = usa_00002, weight = ~PERWT , repweights = “REPWTP[0-9]+”,
type = “JK1”, scale = 4/ 80 , rscales = rep(1, 80 ), mse = TRUE)
I believe the correct syntax for svyttest() would then be:
svyttest(percentRACBLK~COUNTYFIP, svy)
I’m not sure what this will look like with lots of counties. You might need to make a subset of your data with just the two counties. Also note that not all counties are identified in the ACS microdata.