Just to clarify a bit more, you can't draw in BlueGUI windows with the standard DBP commands (as far as I know), you have to use windows gadgets instead. For example, you can use label gadgets to display text or static images on another window.
Ex: For a health bar, make a panel, colour it and resize for correct amount.
startBlue "XXX","XXX"
mainWnd = creategadgetfromwindow(mainwindow())
auxWnd = createWindow(0,0,320,240,"AuxWnd",WINDOW_FIXED,0,0,0)
healthBarPanel = createPanel(0,0,100,20,auxWnd)
setGadgetColor healthBarPanel,rgb(255,0,0),0
Health# = 100
activateGadget mainWnd
do
if upkey() then inc Health#
if downkey() then dec
if upkey() or downkey()
if Health# > 100 then Health# = 100
if Health# < 1 then Health# = 1
resizeGadget healthBarPanel,Health#,20
endif
loop
And you can use buttons with setButtonImage to create things such as inventory selection.