I am not totally sure what you mean by "blanks", so I assumed you meant an empty string. So here is how I would do it:
library(dplyr)
data <- tibble(
x = c('A', '', 'C'),
y = 1:3
)
data
#> # A tibble: 3 × 2
#> x y
#> <chr> <int>
#> 1 "A" 1
#> 2 "" 2
#> 3 "C" 3
data %>%
mutate(
across(1, na_if, y = '')
)
#> # A tibble: 3 × 2
#> x y
#> <chr> <int>
#> 1 A 1
#> 2 <NA> 2
#> 3 C 3