More than you wanted to know about the 2d blues:
1840: Plates 1 and 2 issued. Plate 1 printed stamps that were available in early May. Plate 2 went to press in July. Both are usually cancelled by the Maltese Cross.
Note on the Maltese Cross - it was originally inked in red, but postmasters were instructed to switch to black in in February 1841 (though there were some trials of black ink in London in late 1840.
1841: New 2d plate made - Plate 3 - with white lines at top and bottom. This plate was registered on February 25, 1841 and went to press a day or so later. At this time, the standard cancellation was a black Maltese cross
1844: The Maltese Cross obliterators were replaced by a series of numeral cancellation devices - 5 different styles. Plate 3 continues to be used to produce 2d blue stamps
1849: Plate 4 of the 2d blue is put to press (and plate 3 retired). Note that this is 5 years after the Maltese cross was retired.
1854: Perforated stamps became the norm
Shades: pale blue, blue, dark blue - all the same value. Also, violet blue, which was only printed in 1851 (so therefore only on plate 4), is very scarce, has a catalog value of £1800, and requires certification. Unless you bought it as a violet-blue, it most likely isn't. Save your money on the certification fees unless you're virtually certain.
Therefore:
If it is a 2d blue with white lines and a Maltese cross, it is almost certainly from plate 3
If it is a 2d blue with white lines and a numeral cancellation, it can be from either plates 3 or 4.
As the MX was only used for 3 years on one plate, it has a premium value (SG catalog for an 1841 2d with MX is £250, with a numeral cancellation is £100)
Exceptions:
There are 1840 2d blues (no lines) that have numeral cancellations, but these are the exception and are worth a premium (SG catalog for an 1840 with MX is £850, with an 1844 numeral is £2500)
There may be a plate 4 with a MX, but I haven't seen one.
For more on the imperforate line engraved stamps (black, blue, and red), here's another site with a somewhat quirky collection of them:
http://victoria.cgpostal.com