|Could be many things. If you are seeing sparks on the cathode, then the tester is overloading the tube. If you see a purple glow, then the tube is gassy (though it would also do this on other testers and in radios). Blue on the glass (usually dim, and quite deep in color, though sometimes pale, too) is simply a phenomenon of cathode rays. I do not believe that it has anything to do with gas in the tube (rather the lack of). Seems to almost be residue left on the glass and surfaces, but the way it glows leads me to believe otherwise, since it doesn't seem to 'select' particles or material, though it usually takes on a somewhat regular pattern.
At any rate, most likely your problem is the first one. If so, check for shorts, improper resistors, improper switch settings, etc. If I put a 12AU7 in my EICO 625 and set the SELECTOR switch (as it is labeled) to #4, the tube will spark and flash, and the meter will jump all over the place. The #4 position in my tester uses a very high current low resistance resistor at 200 volts. The rest of the settings use much higher resistances at lower voltages, thus subjecting the tube to less current.
Check resistors, capacitors, wiring, etc.