How do I get blackboard bold symbols?
This question applies only to Lurch 0.75. In Lurch 0.76, some blackboard bold symbols appear in their own palette in the math toolbar.
You can type ℕ, ℚ, ℝ, ℤ and ℂ (or any other blackboard bold character) in Lurch by typing ordinary letters like N, Q, R, Z, and C and then changing their font to jsMath-msbm10. If you want to put blackboard bold characters inside a meaningful expression, you should turn off auto-formatting of the contents of the meaningful expression bubble contents first. To do this first create your meaningful expression and its contents, then right click on the bubble and choose Stop auto-formatting from the context menu. Then you can change characters in your math expression to blackboard bold as described above.
Keep in mind that Lurch does not consider character formatting when determining meaning, so that e.g. the expressions ℕ and N will mean the same thing (assuming you made the ℕ by formatting N with the jsMath-msbm10 font).