Lighter fluid (naphtha), Goof-off, brush cleaner (not to be confused with mineral spirits), or mineral spirits. All should work. Test a spot to make sure whatever doesn't also take up the boot-stripe.
PS - blue tape and masking tape shouldn't be left on for more than three days or so in the sun. There are longer time versions if you think you'll need them.
Charlie P. P31-2 #80 CATNAP
To travel hopefully is a better thing than to arrive - R.L. Stevenson
I suspect that, if you should go to the end of the world, you would find somebody there going farther . . . - H.D. Thoreau