I have a small Neon bulb fitted with a BC base its roughly the size of a 15 watt pygmy lamp my question is I saw another one of these  on youtube on rustymotors channelthat's been run almost continually 24 hour a day for the last 10 years and the glass is badly blackened but why? What blackens the glass?

I had three neon bulbs with a ba15d or something base. They blacken since the bombardment of electrons on the electrodes. Maybe somebody more knowledgable like Ash or Medved could fill you in more.

The electrodes sputters their material, as the lamp ages.

The voltage drop across the discharge breaks down to the voltage (and multiplied by electron's charge thats energy) it takes to pull the electron out of the metal cathode - that is cathode voltage, and rest of the voltage that falls acros the discharge path length

The cathode voltage goes down if the materials let out the electrons with less sffort - thats what emitter coating in fluorescent cathodes is for, and it goes down with temperature - thats why hot cathode lamps are made

The glow lamps dont have any of those advantages. Just electrons pulled out of cold metal by high cathode voltage

The cathode voltage is present over a short distance - the nearest vicinity of the cathode. In this short distance everything that's charged and can move, is accelerated to its full kinetic energy (high voltage over short distance - strong electric field, so big acceleration)

Neon ions and free electrons in this space are accelerated, in oppposite directions - The electrons away from the cathode, Neon ions missing an electron into the cathode. When they hit each other on their way, electrons fly out of Neon ions or move to higher energy state. When they drop back to the lower state, they emit their excess energy as photons, in the wavelengths that match the type of atom .In Neon, the "fall height" in energy that most of them fall through match the Red band. Fewer electrons fall from higher energy states or from completely outside, that would be higher energy bands i.e. UV etc. They are fainter as fewer electrons go this way

Finally some of the accelerated ions (that have not yet wasted their energy by hitting something else) hit the metal cathode. Positive ions are no electrons, their mass is nearly same as complete Neon atoms and the impact from them is quite hard. Occasionally hard to the extent that it kick metal atoms out of the electrode, and those land on the glass

Thank-you it all sounds quit violent I guess it explains why blackened tubes are usually near EOL