I don't know how to set partial text as a Bold to LabelField in Blackberry.
Kindly refer the attached image,
This example should contain everything you'll need. Your code should look something like this:
String copyText = "Tap NEXT to proceed";
Font[] fonts = new Font[2];
fonts[0] = Font.getDefault();
fonts[1] = Font.getDefault().derive(Font.BOLD);
int[] offsets = new int[4];
offsets[0] = 0;
offsets[1] = "Tap ".length
offsets[2] = "Tap NEXT".length;
offsets[3] = copyText.length();
byte[] attributes = new byte[3];
attributes[0] = 0;
attributes[1] = 1;
attributes[2] = 0;
RichTextField textField = new RichTextField(copyText, offsets, attributes, fonts, NON_FOCUSABLE);
add(textField);