index <- seq(1, 10, 1)
blockID <- c(100, 120, 132, 133, 201, 207, 210, 238, 240, 256)
df <- as.data.frame(cbind(index, blockID))

blocklist <- c(100, 120, 130, 150, 201, 205, 210, 238, 240, 256)

df2 <- df %>% mutate(blockmatch = ifelse(blockID %in% blocklist, blockID, ifelse(match.closest(blockID, blocklist, tolerance = Inf), "missing")))

df %>%
  rowwise() %>%
  mutate(
    new = blocklist[order(abs(blockID - blocklist))][1]
  )

# A tibble: 10 x 3
   index blockID   new
   <dbl>   <dbl> <dbl>
 1  1.00     100   100
 2  2.00     120   120
 3  3.00     132   130
 4  4.00     133   130
 5  5.00     201   201
 6  6.00     207   205
 7  7.00     210   210
 8  8.00     238   238
 9  9.00     240   240
10 10.0      256   256