BLACKLINE, INC.

Appendix A

Unaudited Reconciliation of Non-GAAP Financial Measures

(in thousands, except percentages)

(1) Beginning in 2023, includes amortization related to stock-based compensation that was capitalized in capitalized software development costs in previous periods and totaled $2.1 million for the year ended December 31, 2023.

Signature [PLEASE SIGN WITHIN BOX] Date Signature (Joint Owners) Date TO VOTE, MARK BLOCKS BELOW IN BLUE OR BLACK INK AS FOLLOWS: KEEP THIS PORTION FOR YOUR RECORDS DETACH AND RETURN THIS PORTION ONLYTHIS PROXY CARD IS VALID ONLY WHEN SIGNED AND DATED. V35578-P01864 ! ! ! For All Withhold All For All Except For Against Abstain ! !! ! !! BLACKLINE, INC. To withhold authority to vote for any individual nominee(s), mark "For All Except" and write the number(s) of the nominee(s) on the line below. BLACKLINE, INC. 21300 VICTORY BLVD, 12TH FLOOR WOODLAND HILLS, CA 91367 01) Owen Ryan 02) Sophia Velastegui 03) William Wagner Nominees: 3. Approval, on a non-binding, advisory basis, of the 2023 compensation of the Company's named executive officers. 2. To ratify the appointment of PricewaterhouseCoopers LLP ("PwC") as the Company's independent registered public accounting firm for its fiscal year ending December 31, 2024. Please sign exactly as your name(s) appear(s) hereon. When signing as attorney, executor, administrator, or other fiduciary, please give full title as such. Joint owners should each sign personally. All holders must sign. If a corporation or partnership, please sign in full corporate or partnership name by authorized officer. NOTE: In their discretion, the proxies are authorized to vote upon such other business as may properly come before the Annual Meeting of Shareholders or any adjournments or postponements thereof. 1. Election of Directors The Board of Directors recommends you vote FOR the following: The Board of Directors recommends you vote FOR proposals 2 and 3. VOTE BY INTERNET Before The Meeting - Go to www.proxyvote.com or scan the QR Barcode above Use the Internet to transmit your voting instructions and for electronic delivery of information up until 11:59 p.m. Eastern Time on May 8, 2024. Have your proxy card in hand when you access the web site and follow the instructions to obtain your records and to create an electronic voting instruction form. During The Meeting - Go to www.virtualshareholdermeeting.com/BL2024 You may attend the meeting via the Internet and vote during the meeting. Have the information that is printed in the box marked by the arrow available and follow the instructions. VOTE BY PHONE - 1-800-690-6903 Use any touch-tone telephone to transmit your voting instructions up until 11:59 p.m. Eastern Time on May 8, 2024. Have your proxy card in hand when you call and then follow the instructions. VOTE BY MAIL Mark, sign and date your proxy card and return it in the postage-paid envelope we have provided or return it to Vote Processing, c/o Broadridge, 51 Mercedes Way, Edgewood, NY 11717. SCAN TO VIEW MATERIALS & VOTEw

V35579-P01864 Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting: The Notice and Proxy Statement and Form 10-K are available at www.proxyvote.com. BLACKLINE, INC. Annual Meeting of Shareholders May 9, 2024 9:00 a.m. PT This proxy is solicited by the Board of Directors The shareholder(s) hereby appoint(s) Mark Partin, Chief Financial Officer, and Karole Morgan-Prager, Chief Legal and Administrative Officer and Secretary, or either of them, as proxies, each with the power to appoint his or her substitute, and hereby authorize(s) them to represent and to vote, as designated on the reverse side of this ballot, all of the shares of Common Stock of BLACKLINE, INC. that the shareholder(s) is/are entitled to vote at the Annual Meeting of Shareholders to be held at 9:00 a.m. PT on May 9, 2024, at www.virtualshareholdermeeting.com/BL2024, and any adjournment or postponement thereof. This proxy, when properly executed, will be voted in the manner directed herein. If no such direction is made, this proxy will be voted in accordance with the Board of Directors' recommendations. Continued and to be signed on reverse side