Merge 2 lines with delimiter

[Buddhism]

So I have code that prints out a into a file like this:

xxx.xxx.xx.xx
yyyy
#5 blank line gap
xxx.xxx.xx.xx
yyyy
#5 blank line gap
#This repeats for a while

I need to merge the 'x' and 'y' with a ':' delimiter

So end file will look like:

xxx.xxx.xx.xx:yyyy
xxx.xxx.xx.xx:yyyy
xxx.xxx.xx.xx:yyyy
xxx.xxx.xx.xx:yyyy
#etc.