You're right, it is more likely the dealer will make a pat hand. From my blackjack appendix 2, the following are the probabilities of the dealer's final total given a 5 as an up card. This assumes the dealer stands on a soft 17, which I believe is what you do.
17: 12.23%
18: 12.23%
19: 11.77%
20: 11.31%
21: 10.82%
bust: 41.64%
Assuming 8 decks, there are 16*8=128 10 point cards in the deck. Eliminating the ace there are 52*8-1=415 possible cards under the ace. Thus the odds of a blackjack are 128/415 = 30.84%.