The booking class is the basis for the fare you paid.
Within the 'economy' ticket section, there are different fare "classes", A, B, C, D, etc, that have different price levels and a certain number of tickets allocated. Different fare classes can also affect what types of rights you have - like if it's refundable or changeable.
So you could have 20 tickets in class D at €100, 30 tickets in class C at €150, and so on. But perhaps also buying class C means that you're allowed to change the date of your ticket.
Additionally, some types of booking class are ineligible to earn miles, generally the very cheapest.
The pricing for tickets is not alphabetically ordered. For most airlines F means full-fare first class, Y means full-fare economy or economy plus/extra, and C or J can be full-fare business.
According to this list, the Blue1 booking classes: G/K/L/O/T/U/W are not eligible for miles.
If you order online you should have received an email confirmation with an invoice/receipt that states your booking class somewhere and the level of refunds/changes you can receive. I'd check your booking confirmation first.