Sometimes the clear Juvederm gel can dissect through the tissue planes and move from deeper placement to more superficial locations (just below the skin) where the Tyndall effect makes the "blister-like" areas look bluish in color. Though injection with hyaluronidase can be performed, it actually breaks down native tissue hyaluronic acid more than hyaluronic acid fillers (which are more cross-linked, and which is why they "last" in tissues). The effect of diminishing the bleb, bump, or "boil" may be the same, but it usually is incomplete.
Better, in my opinion, is using a sterile needle to puncture the Juvederm bubble, gently squeeze the excess out, and massage the rest into the soft smooth look you wanted in the first place. Consider slightly deeper placement the next time, or perhaps a different injector if this seems to be a recurring problem.
BTW, arnica is a great homeopathic remedy for bruise reduction and reabsorption (we use it in my surgical practice daily because we feel it really works), but it does not prevent bruising. If your injector hits a vessel (we all do on occasion, because they are there!), arnica or not, you WILL get a bruise. Just a smaller one, and one that goes aways faster, but a bruise just the same!