+1 on what milepig suggests. That's what I always do (check in online for UA900 in united.com; and check in for the LH flight in lufthansa.com; if you need to check bags, check them with United in SFO.
Now, in your flight back, you encounter a similar problem; but you deal with it differently. At BLR you need to check in and drop bags at the airport with LH but (different than SFO) the agent will have no problems printing your boarding passes for your United flight from Frankfurt. However, when you get to the gate at Frankfurt, a United agent will exchange your LH-printed boarding passes for a boarding pass printed on UA stock.
Originally Posted by
milepig
Lots of options - especially since you're 1K.
UA *may* be able to print your LH BP, but I've had very mixed success.
By far the easier route is to checkin online with LH directly - an added advantage is that as *Gold you'll also be able to select a seat then. Your record at ua.com will have a link to your LH PNR, which is what you'll need for the LH system.
Or, you can do it at the Senator Lounge in FRA, even if you've checked in already, have them check it at the lounge and (in my experience) they may reprint it. If you have checked bags they'll also enter that into their system, which will save time at the gate.
I have NOT had good luck checking in for a LH flight at a non-originating airport (as in at ORD for a FRA-XXX flight) they told me it couldn't be done and to wait until I got to FRA. This is why Online is my preferred option.