Don't build dynamic variables; keep your data out of your variables. Build a list or dictionary instead.
With a dictionary, for example, it'd be:
card_labels = {}
for i in range(2):
for j in range(6):
label = CardLabel(root)
label.grid(row=i, column=j)
label.configure(image=CardLabel.blank_image)
card_labels[i, j] = label
This stores the labels keyed on the (i, j)
tuple in the card_labels
dictionary.