With awk:

awk -F: '{print $1$2"."$3$4"."$5$6}'

-F: will set : as the field separator and print $1$2"."$3$4"."$5$6 will print fields $1 and $2 then print a . and so on ... the rest is self explanatory.

Example:

$ echo "bl:ab:la:bl:ab:la" | awk -F: '{print $1$2"."$3$4"."$5$6}'

blab.labl.abla

It can also read from a file like so:

awk -F: '{print $1$2"."$3$4"."$5$6}' file

and modify the file in-place(i.e. edit the original file) like so:

gawk -i inplace -F: '{print $1$2"."$3$4"."$5$6}' file