White lines appearing between coloured blocks in test email

I'm encountering a very frustrating glitch in which there are white lines showing between coloured blocks grouped together. For example- I'll have a coloured block with two spaces on the top and bottom, all dark blue. What should be all dark blue has thin white lines where the edge of the different blocks are. I cannot change the entire email background to this colour as it's the same as my heading titles. 

The white lines do not show up in the editor or preview, only when the email is actually sent out. Tried multiple workarounds and reset all the colours and it's still not fixed. 

Help! It looks terrible.