Is the 64-bit blocksize a fatal issue when encrypting TBs of data with Blowfish CBC?
I didn't find anything about the exact way Crashplan encrypts files, only that it uses Blowfish in CBC mode. The block size of Blowfish is 64 bit, so there are $2^{64}$ different input blocks and the same number of output blocks. All in all $147573953$ terabytes of different output data. The problem with this is the birthday attack. Summarized it says that the problems with too much encrypted data (for CBC mode) start at ca. $\sqrt{2^{n}} = 2^{\frac{n}{2}}$ (n denotes the block size in bit). For Blowfish we get approximate 34 gigabyte. You'r mentioned "TBs of data" are much more than this.
The accepted answer from "poncho" to the question "Importance of block size in CBC mode" shows why it is no good idea to encrypt much more than the mentioned limit with a single key. It would be a good idea to change the algorithm to a higher block size variant or change the key after ca. 34 gigabyte or even less.

To analyse the ciphertext of Crashplan you have to know how the program saves the encrypted data. Try to convert the saved file into the original output blocks of Blowfish and then look for equal blocks. As soon as you find two equal blocks you can use the attack in the answer of poncho mentioned earlier. If you know the plaintext block of one of the block (due to some patterns or something else) you can calculate the value of the other block.

How bad is this attack? Well, the attacker has to have good luck with finding equal blocks. The bigger the ciphertext, the higher the probability for equal blocks. After that the attacker has to know some patterns of the plaintext. Many identical or just similar files (like many pictures saved in PNG format) can help the attacker to exploit patterns. And that's only for the hardest attack someone could use, a Ciphertext-only attack (the enemy only knows the ciphertext, but nothing concrete about the plaintext). If the attacker has the ability to save his or her own files in your backup (like a employee maybe could do it) there's no way to prevent him or her from getting special patterns into your plaintext.

There's no publicly known way to break Blowfish. Even if your key is a weak key (or will be hashed into a weak key) it won't harm the encryption in any known way. The mentioned attacks only work for a reduced-round Blowfish.