Hi Roy
It can be very difficult to date antique coloured glass. Particularly 'Bristol' blue. The early 19th century saw the most prolific production of wrythen blue glass items and they were commonplace as milk/cream jugs and sugar bowls from around 1800 through to 1840 or thereabouts; though friggers in all forms were produced, to include bugles and all manner of fine forms that demonstrated the glass blowers skill. It is easier when there are complimentary dating features such as the shape of a handle etc; or engraving or gilding (that may sway you in the wrong direction?). Blue wrythen items were produced in all shades of blue from a very weak blue through to the traditional shade we recognise as 'Bristol' blue during the period I have mentioned. That said; they were produced later, again in all shades and in all forms; and still are currently. The identifiers I would be looking for are signs of period glass production and this would apply to any coloured or clear glass: Striated (swirly lines in the glass). Seeds (unmelted silica etc). Inclusions (pieces of production 'muck' in the glass from relatively primitive working conditions) and wear. Very few items from this period are signed or have definative identifiers and those that are are extremely valuable. The prescence of the rough pontil as I am sure you are aware just identifies that it has been held by a punty at some point and therefore hand made; not an identifier of age. My advice is to look for the age adentifiers I have mentioned and if you find none; then consider it relatively modern. A good piece nonetheless.
Regards. Trev.