The color blue is associated with boy babies, at least in the US.
In the US, blue is considered the stereotypical color for boy babies, and pink for girl babies. This is expressed often in boys being given blue things to where; while girls are given pink things to wear.
In regards to cutting a cake specifically, this was an example of a "Gender reveal ...
The show's creator, Dan Fogelman, spoke to The Hollywood Reporter
Was the three-part episode something you plotted out from the beginning or was it also a helpful way to make sure Jack's death fell on the Super Bowl episode?
"No, we always knew that. We didn't know how many episodes that would fold out on, but making Jack's reveal be [...
Olivia's Mother is American, but she abhors Thanksgiving because she doesn't want to get together with "relatives she hates so they can gorge themselves on dry bird while pretending that they'd ever have anything to do with eachother if they weren't related by blood". Her plans, instead, are to go to a favorite dive bar.