Though I would open up a new recent topic on this to facilitate searching as the problem seem not to have been completely addressed by Garmin as of 2018. Here's how I solved the problem of blue pins being used for via marks on my eTrex 30x. These pins cause a lot of clutter when trying to read the route on the eTrex screen. The problem is due to symbols not being universally recognized across all Garmin software and devices and the blue pin is subsequently used as a default symbol.
To replace the blue pin with something not nearly as cluttered,
I used the paint application in Windows 10.
1) edited the colors to create magenta
RGB= 255, 0, 255
2) Selected the color for both color 1 and 2
3) Created a rectangle and muddled my way through paint to get the size down to below 32 pixels per dimension (Careful, I missed this in my first attempt and it didn't work because the pixel dimensions were over the limit)
4) Saved as file type 24-bit Bitmap with the name "Pin, Blue.bmp" to the CustomSymbols folder on the eTrex
The blue pin is now replaced by the small magenta square.
The solution works but does take some perseverance to successfully complete.